I replaced mine with a firm, full size house-bed mattress (same length as the crappy original, just inches narrower), and put a 3 inch foam rubber/memory foam topper underneath it to simulate the give of a box springs. Has worked well for me for a year and a half so far. No more backaches in the morning.
Edited to add: sorry, I didn't catch that it was for your dinette bed. I was talking about the short queen bed in mine. Not sure how to fix your problem. Maybe a good quality air mattress would work.
2011 Dodge Ram 1500 HEMI Quad Cab
2011 Jay Flight 26BH