Gary & Eileen,
We replaced our mattress with a dual-chamber air mattress when I was having severe back problems while on a trip. I was going to buy a Select Number mattress one like we have at home (just a different size), but they were not available in stock in the state we were passing through. So I bought a knock-off brand. A queen size fitted our coach just fine, but you should check the size of your mattress. A queen is actually 1-2" longer than the original mattress in our coach, but there was enough room to accommodate the additional length with the bedroom slideout retracted.
If you go this route, remember that you only need the mattress and don't need the foundation. Because of my time constraint, I bought both and dumped the foundation. Actually, I took the old foam mattress (it was only a few weeks old) and gave it and the new foundation to one of the kids for their guest bedroom.
My back problem went away.