Fleetwood Jamboree matress

We had one made by a mattress company that makes standard & custom mattresses. Our rv queen corner cut is now very comfortable. There are companies like this in most large cities, ours was made and installed in Reno. Dick

The mattress that came in my 'C' when it was new, was horrible. It was a short queen, on top of a platform that included the lid to the storage area at the foot of the bed.
I went to "The Original Mattress Factory" store near me. The salesman had me measure the distance from the top of the platform to the hinged storage lid and total length. They built an all in one mattress/box spring combo with a hinge where the storage box opened. Same price as a name brand short queen/box spring.
Unfortunately, they're only in 9 states, but it might be worth the drive. FL, GA, KY, MN, NC, OH, PA, SC, and VA.
BTW, to keep the storage box lid open while I rooted around, I bought a couple of crutch tips and a 6 foot dowel to match the inside diameter of the crutch tips. Easy to raise the lid and prop it open with the sticks while I root around for something.

There is always PPL

Aren't special RV replacement mattresses available? I would be surprised if Fleetwood, etc., can't supply one.

Measure the space the mattress takes up , most ARE NOT a standard double, queen size , they are an oddball size !

There are several places that sell them.

Click here for one

Just buy a decent memory foam mattress and cut the corner off.

Not exactly what you're asking - but our 2008 has one radius corner on a full-size queen. I've googled and priced a few replacements have have decided I will not replace it with the same but take one of two alternate approaches: buy a std queen and live with the square edge or buy 4-6" of good foam from Walmart/Amazon and trim it with a knife.