ok so not a "law" or "rule" just a balance between weight and convenience.
i own three rv sized tanks, two in use third as a extra. and so far not needed the third. but a friend needed it.(he burned up both of his smaller tanks). and forgot it was there to use, he froze trying to stay warm in his pu truck. or even using my rv, i was gone for a couple days and a quick snow storm hit.
just with all the space on the larger fifth-wheels, one could easily install four tanks. or even a built in tank. back to weight here.