Most RV mattresses are odd sizes (short queen is popular for bedrooms, but overhead bunks might have something bigger than a super king, and folding mattresses are starting to show up). Many mattress manufacturers can provide the short queen, but suppliers of some of the other oddities might be limited.
Cheap mattresses are not universal, although common at entry RV price levels; Winnebago provided my RV with a hotel-grade Serta Perfect Sleeper, short queen size. But even if the mattress is high quality, it might not be the right mattress for you. Find out the size you are dealing with, then shop mattress stores and factory outlets for something that suits you, and have it made to fit.