Both of the following sites offer custom made RV mattresses. I have not purchased anything from either one so I can only offer them as a resource to check.
The following site overs mattresses but also offers custom sewing. this may be the site you are looking for if you want a custom made cover.
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 how either 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,