My brother recently bought a 42' sailboat. We were discussing all the things he bought and replaced and the similarity between RV stuff and boating stuff. In a boat, they often have a "V" berth comprised to two cushions making up the bed. He bought a fitted mattress topper made of tufted material, not foam to cover the "V" berth cushions. He said the cover was extremely comfortable.
You contact the company, they send you a kit to make a template of your mattress and then they produce a fitted mattress topper. It's quilted, so it's comfortable and doesn't get hot like foam.
What was really amazing is that they will take your sheets or theirs and fit the to your mattress. I thought this was the perfect company for those who are looking for a mattress topper that fits and sheets that fit.
Here is the website....tuftedtopper: Products
. Even though they primarily talk about boats, they do motor home mattresses too. They're probably a little more expensive than some of the toppers, but you're getting a custom fit.
I have no affiliation with this company, but thought it was a great place for anyone who wants to add a topper, a subject that comes up here all of the time.