The mattresses that come with an RV are terrible. A dog would probably choose to sleep on the hard floor.
I bought my replacement from a place called "The Original Mattress Factory" in a nearby strip mall. The salesman had me measure from the headboard to the hinge of the under bed storage space. They built me a single mattress/box spring combo with a wood frame hinged on the bottom so that I could get to the storage area easily. No more expensive that a good quality regular mattress.
Unfortunately, they're only located in 9 eastern states.
If you google "Original Mattress Factory", there's a list of states.
Fortunately, for the OP, there are several in North Carolina.