Have a look at memory foam mattresses. You can likely use the cover from your old mattress and just cut the new one to fit.
We bought an 8 inch one from Costco, delivered to our door. It's a full length and is compressed to RV Queen size when the slide is in, but it's nice to not have my toes hanging out the bottom end of the bed anymore.
Happy Glamping, Norman & Elna. 2008 Winnebago Adventurer 38J, W24, dozens of small thirsty ponies. Retired after 40 years wrenching on trucks! 2010 Ford Ranger toad with bicycles or KLR 650 in the back. Easy to spot an RVer, they always walk around with a screwdriver or wrench in one hand!