Many places make custom mattresses, especially in foam. I had an innerspring one made for my 22ft CCD. What I learned from the experience:
1. good innerspring is HEAVY, hard to make up in confined area
2. Heavy mattress = less hauling capacity
3. Don't like foam? Try several types, they aren't all alike.
4. Memory foam? some love 'em. I don't like to climb UP to turn Over.
5. Inflatables? Possible, Some good and $$$, cheap OK for part time.
6. Mattress toppers? GOOD, but consider foam or fiberfill.
I had featherbed, I had feathers everywhere! Must pay top dollar to get one that doesn't shed itty bitty feather pieces. (Fiberfill is WASHABLE).
Custom is generally only 5% to 10% more than standard.
Old airstreams had sandwitched foam. Use "Search" - there are many good threads from those who have gone before.