John-boy,
The following sites offer custom made RV mattresses. I have not purchased anything from any of them so I can only offer them as a resource to check.
http://www.foamsource.com/overview.php/9 http://www.sdiproducts.com/sdiproducts/rvmattress.html http://www.foamorder.com/
Some threads that discuss mattress options are:
"Do you know of a source for replacement mattresses?"
"Ever use a foam mattress pad?"
"Airstream twin Bed solutions"
I would be interested to know if one of them works out since I will eventually need to do something for my AS. Of couse others might be interested too.
Hope this helps some,
Malcolm