SUPPORT SYSTEM FOR AN ALTERNATIVE FUEL HEAVY VEHICLE CROSS-REFERENCE TO RELATED APPLICATIONS This Patent Application claims priority from Italian Patent Application No. 102021000001037 filed on January 21, 2021, the entire disclosure of which is incorporated herein by reference. TECHNICAL FIELD The present invention concerns a support system for a vehicle, in particular support system for an alternative fuel heavy vehicle such as a fuel cell vehicle. BACKGROUND OF THE INVENTION Alternative fuel heavy vehicle needs to house heavy and delicate elements such as batteries or fuel cell devices to allow the traction of the vehicle. Such elements may be housed in the front portion of the vehicle, i.e. in the space traditionally occupied by the internal combustion engine, or in a rear portion of the vehicle. In any case, they are housed between the longitudinal members of the frame and supported by these latter. However, it is known that the frame has a rigidity that may be increasingly varied by the fixing of the alternative fuel elements thereby leading to vehicle stability and maneuverability problems. Moreover, the torsional moments and longitudinal accelerations may damage the alternative fuel elements carried by the frame. Accordingly, the need is felt to provide a system for supporting alternative fuel elements in order to avoid the aforementioned drawbacks. An aim of the present invention is to satisfy the above mentioned needs in a cost effective and optimized way. SUMMARY OF THE INVENTION The aforementioned aim is reached by a support system and a vehicle as claimed in the appended independent claims. Preferred embodiments of the invention are realized according to the claims dependent on or related to the above independent claim. BRIEF DESCRIPTION OF DRAWINGS For a better understanding of the present invention, a preferred embodiment is described in the following, by way of a non-limiting example, with reference to the attached drawings wherein: · Figure 1 is a perspective view showing a portion of a vehicle, with parts removed for sake of clarity, comprising a support system according to a first embodiment of the invention; · Figure 2 is a top view showing the vehicle of figure 1; · Figure 3 is a perspective view showing an enlarged view of an element of the support system of figure 1; · Figure 4 is a perspective view showing a portion of a vehicle, with parts removed for sake of clarity, comprising a support system according to a second embodiment of the invention; · Figure 5 is a top view showing the vehicle of figure 4; · Figure 6 is a perspective view showing an enlarged view of an element of the support system of figure 4; · Figure 7 is a perspective view showing a portion of a vehicle, with parts removed for sake of clarity, comprising a support system according to a third embodiment of the invention; · Figure 8 is a top view showing the vehicle of figure 7; · Figure 9 is a perspective view showing an enlarged view of an element of the support system of figure 7; and · Figure 10 is a perspective sectional view of the element of figure 9. DETAILED DESCRIPTION OF THE INVENTION In the attached figures, it is shown a portion of a frame 1 for a heavy vehicle (not shown for sake of clarity) such as a commercial vehicle, e.g. a truck. As known, the frame 1 comprises a pair of side members 2, 3 extending parallel to a longitudinal axis A of the vehicle and laterally spaced one with respect to the other. Always as known, the two side members 2, 3 are connected by at least a cross member 4 extending transversally with respect to axis A and fixed to its extremities to the side members. The cross member 4 may be of any typology and shape according to the dimension and load of the vehicle. According to the invention, the frame 1 is provided by a support system 5 for supporting an alternative fuel module 6, e.g. a fuel cell module, configured to provide the energy needed to the operation of the vehicle. In particular, support system 5 comprises a first support element 7 and a second support element 8 that are configured to connect the alternative fuel module 6 to chassis 1 via a three-points connection. In the described embodiments the second support element 8 is placed in a front portion of the vehicle along axis A and according to its motion direction and the first support element 7 is placed in a rear portion. Clearly, they may be inversed according to vehicle necessity. In particular, the first support element 7 connects a coupling portion, fixable to the alternative fuel module 6, via a movable connection to the cross member 4 while the second support element 8 connects a coupling portion, fixable to the alternative fuel module 6, to the side members 2, 3. In detail, according to all the provided embodiments, the second support element 8 is a transversal member 9 comprising a pair of lateral elements 9a and an intermediate member 9b. The lateral elements 9a defines a cantilevered support portion 9a' that is supported by a flanged portion 9a'' fixed to the respective side member 2, 3. In particular, the flanged portion 9a'' is fixed to the side member 2, 3 via fixing means such as threaded elements, e.g. bolts. The intermediate member 9b is fixed on the cantilevered support portion 9a' of the lateral elements 9a via fixing means such as threaded elements, e.g. bolts. In particular, the intermediate member 9b comprises a coupling plate 9b' that is configured to be coupled, e.g. via fixing means such as threaded elements, to the alternative fuel module 6. The coupling plate 9b' is substantially plate and extends on a vertical plane, i.e. a plane orthogonal with respect to longitudinal axis A. Moreover, the coupling plate 9b' has a central portion that is substantially rectangular, e.g. squared and a pair of lateral portion that are tapered in a way to decrease their thickness when approaching to the respective side member 2, 3. In particular, the lateral portions are inclined with respect to axis A while the central portion is perpendicular to this latter (see figures 2, 5 and 8). The intermediate member 9b furthermore comprises a pair of reinforcing plates 9b'' configured to strengthen the coupling plate 9b'. In particular, each reinforcing plate 9b'' comprises a substantially triangular plate that extends, tapered, from the respective side member 2, 3 towards the central portion of the coupling plate 9b'. Preferably, the reinforcing plates 9b'' and the coupling plate 9b' are made as a single piece. Moreover, it is noticed that the reinforcing plates 9b'' are placed in a plane perpendicular with respect to coupling plate 9b', i.e. parallel to the plane containing the cantilevered portion 9a' of lateral elements 9a. Accordingly, the intermediate member 9b is fixed to the flanged portion 9a'' via the respective reinforcing plate 9b'' preferably via a threaded connection, e.g. by bolts. According to a first embodiment, as shown in figures 1 to 3, the first support element 7 comprises a coupling plate 11 configured to be fixed to the alternative fuel module 6 and a bearing support 12 configured to be fixed to cross member 4 and to support in a rotatably free manner the coupling plate 11. In greater detail the coupling plate 11 may be fixed to the alternative fuel module 6 via fixing means such as a threaded connection, e.g. by bolts and is coupled to bearing support via a shaft 13 that is fixedly engaged to the coupling plate 11. In particular, the shaft 13 extends perpendicularly from plate 11 on the opposite side with respect to alternative fuel module and has preferably a cylindrical shape, more preferably hollow. Preferably, shaft 13 and plate 11 are monolithic and may be furthermore connected together via ribs 14 configured to increase the coupling of shaft 13 with plate 11. The support bearing 12 preferably comprises a plummer block 15 defining an opening 16 wherein the shaft 13 may be accommodated and supported in a rotatably free manner, as per se known. Preferably the plummer block 15 has a “Omega” shape, i.e. comprises a pair of legs 17 configured to allow the fixation of plummer block 15 to the cross member 4. According to a second embodiment, as shown in figures 4 to 6, the first support element 7 comprises a coupling plate 11 configured to be fixed to the cross member via a pair of torsion walls 20', 20''. In greater detail the torsion walls 20', 20'' extends along axis A from coupling plate 11 in a direction opposite to the second support element 8. Preferably, torsion walls 20', 20'' are realized as one piece with coupling plate 11. Preferably one 20' of the torsion walls extends inclined with respect to axis A while the other 20'' extends parallel to this latter. Accordingly, the two plates 20', 20'' are not parallel one with respect to the other and they are closer in their respective portion positioned above the cross member 4. Each torsion wall 20', 20'' furthermore comprises a coupling portion 21', 21'' that extends over the cross member 4 and is configured to allow the connection of each torsion wall 20', 20'' to the cross-member 4. In particular such coupling portion 21', 21'' is a flat portion configured to allow the connection via fixing means, e.g. defining holes that may house threaded elements such as bolts. Each torsion walls 20', 20'' may further comprise a bent portion 22', 22'' that extends along an upper edge of the torsion wall 20', 20'', preferably along its entire extension. Such bent portion 22', 22'' extends inclined with respect to the torsion wall 20', 20'', in particular, in a direction opposite to the torsion walls 20', 20''. In particular the shape may be varied to increase the stiffness of the torsion walls 20', 20''. The torsion walls 20', 20'' have a general tapered cross section, in particular their thickness in a vertical direction decreases from the coupling plate 11 to the coupling portions 21', 21''. In particular each torsion wall 20', 20'' is shaped with an intermediate groove 23', 23'' that divides a wider vertical portion with respect to a thinner vertical portion of the torsion walls 20', 20''. It is noticed that torsion walls 20', 20'' and coupling plate 11 may be realized as a single piece, e.g. via a single metal sheet or by forging. According to a third embodiment, as shown in figures 7 to 9, the first support element 7 comprises a coupling plate 11 configured to be fixed to the cross member via a pair of torsion walls 30', 30'' and a ball joint 40. In greater detail the torsion walls 30', 30'' extends along axis A from coupling plate 11 in a direction opposite to the second support element 8. Preferably, torsion walls 30', 30'' are realized as one piece with coupling plate 11. Preferably the torsion walls 30', 30'' are parallel one with to the other, i.e. they are symmetric with respect to axis A. Each torsion wall 30', 30'' furthermore comprises a coupling portion 31', 31'' that extends over the ball joint 40 and is configured to allow the connection of each torsion wall 30', 30'' to the ball joint 40. In particular such coupling portion 31', 31'' is a flat portion configured to allow the connection via fixing means, e.g. defining holes that may house threaded elements such as bolts. Preferably, the coupling portions 31', 31'' extends from the torsion wall 30', 30'' inclined with respect to axis A and at different heights so that they are superimposed one with respect to the other. In particular, in the disclosed embodiment, the coupling portion 31' of one 30' of the torsion walls is above the coupling portion 31'' of the other 30'' of the torsion walls. Each torsion walls 30', 30'' may further comprise a bent portion 32', 33'' that extends along an upper edge of the torsion wall 30', 30'', preferably along its entire extension. Such bent portion 32', 32'' extends inclined with respect to the torsion wall 30', 30'', in particular, in a direction opposite to the torsion walls 30', 30''. Shape of bent portion may be varied to increase the stiffness of torsion walls 30', 30''. Each torsion wall 30', 30'' is shaped with an intermediate groove 33', 33'' that divides a wider vertical portion with respect to a thinner vertical portion of the torsion wall 30', 30''. It is noticed that torsion walls 30', 30'' and coupling plate 11 may be realized via a single metal sheet. The ball joint 40 comprises an upper portion 40' that is fixedly carried by coupling portions 21', 21'' and a lower portion 40'' that is carried by cross member 4. One between the upper or the lower portions 40', 40'' carries a sphere 41 that is configured to be housed in a seat 42 realized in the other between the upper or the lower portions 40', 40''. In particular, as disclosed, the sphere 41 may be carried by the upper portion 40' and the seat 42 may be carried by the lower portion 40''. The operation of the support system 5 as described in the above embodiments is the following. In general, in all embodiments, the second support element 8 supports the weight of the alternative fuel module 6 while the first support element 7, while supporting the weight of the module, further avoids that torque and/or acceleration transmitted by frame 1 may damage alternative fuel module 6. According to the first embodiment, the support bearing 12 allows the possible rotation of alternative fuel module 6 and its partial movement along axis A thereby absorbing such forces and torques transmitted by frame 1. According to the second embodiment, the torsion walls 20', 20'', thanks to their geometric shape, allows the possible rotation of alternative fuel module 6 and its partial movement along axis A thereby absorbing such forces and torques transmitted by frame 1. According to the third embodiment, the torsion walls 30', 30'', thanks to their geometric shape and the presence of ball joint 40 together allow the possible rotation of alternative fuel module 6 and its partial movement along axis A thereby absorbing such forces and torques transmitted by frame 1. In view of the foregoing, the advantages of the support system according to the invention are apparent. The proposed support system 5 provides a cost-effective and weight-reduced three point fixation to the alternative fuel module. Thanks to such fixation, the alternative fuel module is supported in a decoupled way with respect to the frame 1 thereby avoiding that accelerations and torque may be transmitted to the alternative fuel module. Moreover, the proposed support system may be sized for different dimensions of vehicles and alternative fuel module, therefore it is versatile. It is clear that modifications can be made to the described support system which do not extend beyond the scope of protection defined by the claims. For example, as said, first and second support elements 7, 8 may be placed, inversed, in front or rear portion of the vehicle along side members 2, 3. Generally the proposed shapes of the first and second elements may vary according to the dimensions and typology of vehicle and of the alternative fuel module to be supported. The described embodiments may be combined together, i.e. the torsion walls described may vary their shape or be used in combination with a shaft supported to a plummer block or the ball joint may be used in combination with different typology of torsion walls or alone to support directly the coupling plate 11.