I have had a very good experience with a memory foam topper; it transformed the original "mattress" into a quite comfortable mattress.
If the RV has a nonstandard sized bed, a memory foam topper can be cut very easily with an electric knife (the turkey carving kind) or only slightly less easily with a serrated bread knife. I used the latter and it took maybe two or three minutes to transform the queen topper into a short queen topper plus a strip of foam.