You are somewhat limited by the closed up space. We used about a 1.5" mattress topper on ours for much improved comfort. Bought a King and it was bit larger, but the extra just wrapped and tucked under the front.
I also just recently read of a hack for adding the rubber comfort floor tile panels beneath the mattress up against the wood for both comfort and improved insulation. Easily cut to fit, and really cheap at Home Despot/Lowes. Thinking of trying this on our current HTT which has similar bed setup to your pup.
2011 Jayco X19H (purchased 2015)
2008 Jayco 1007 PUP (purchased new, traded for the X19)
2018 Nissan Titan Midnight Ed.