I ended up getting one of these to replace the factory short queen:
Parklane Mattresses 60" x 74" The Explorer" Short Queen RV Mattress
https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1
Seven months later, I still have no regrets about that decision, although the bar wasn't set that high since the original was so useless that I could feel the plywood underneath. I decided on springs rather than memory foam because I heard that the foam deformed over time. True or not, I can still always count on a good night's comfortable sleep on this one.