Slide Out:
If you’ve got a slide out in the main bedroom, often times that will require a shorter mattress (ex: RV Short Queen) to prevent the slide from crushing the bed. If you’ve got a slide out and wish for a longer mattress, one way around that would be to get a hinged mattress towards the foot end of the bed. When closing the slide out, you’ll just need to fold the mattress up at the hinge.
Supporting Surface Length:
Another scenario is when the surface the mattress is going on, often times plywood, isn’t long enough to accomodate a regular size bed. If you’ve got adequate space in the room, you can fix that issue by putting down a longer sheet of plywood.
Small Bedroom:
Many RV bedrooms have barely enough room to walk around the mattress and you’ve just got no other choice but to put a shorter mattress in your RV.