I hear ya! We have a 2007 Winnebago Sightseer 35J. We went with the Costco memory foam topper, which made it seem better but in all reality it still sucks.
With the slide coming in against the mattress I've been aware I need to watch the length of the mattress. If you got with a more traditional mattress get your measurements, you might need to go with a RV mattress which tend to be shorter.
We have a SleepNumber at home, good bed but really don't want to put that kind of money into the RV mattress.
After reading a lot of very positive reviews and talking to owners we've narrows down to the Wal-Mart Spa Sensations Memory Foam Mattress, I need to verify the 76" x 80" but appears to be the right balance of features & cost. Might want to check them out; different sizes range from 6, 8, 10 or 12" height.