Seanmac, another site I check on had an article about rv mattresses. In the article it mentions 2 makers of rv mattresses. Tochta or Beloit, do a google search and one makes specific size mattresses. I don't remember prices, so if its too much $$, check on futon mattresses. That's what I currently use on my back bed. Had to trim the foam a bit, but the new foam fit the original cover and I've been happy with it. Hope this helps ya.
https://www.rvtravel.com/rv-mattress...ing-tips-2306/