Getting ready to order this (5.5" Elation Cool-MAX® Gel Foam RV Mattress II) for about $350 from Mattressinsider.com. Bought my last mattress from them and was comfortable for the money. Mattress toppers don't seem to work on really cheap mattresses, for me anyway. Last one I bought from them was comfortable, but of course, it was a different size than my new camper.
Gotta have a decent bed.
Mark and Joanne
2014 Fun Finder XT 276
2015 Ford F150 Lariat 3.5 Ecoboost 4X4 SCrew Max Tow
2011 Harley Road Glide Ultra