I would try Sleep Train, Mancini's or another similar mattress dealer. I'm not sure of mattress sizes. There are so many different sizes used in RVs. If 72x80 is a standard size, you are in luck. Is it on springs or on a board?
If on a board, I would test out the mattress I was considering on a board or the floor because they feel a lot different on top of a box spring set.
2004 Tiffin Allegro 27.5 ft. P32 18,000 lb GVW. 8.1 liter. Workhorse chassis built May 2002. 21,000 miles. 2012 Jeep Liberty Toad. RVi2 brake unit.