Do you have an Original Mattress Factory or similar nearby? Since they actually make the mattresses there, you can get pretty much what you want. They made us a short queen foam for our last RV and it was great.
I had a queen short mattress in my motorhome, replaced it with a 10" memory foam from Amazon, love it. I have slides on both sides of my coach. The mattress hangs off the end a little when the slide outs are out. Doesn't make a difference sleep wise but the top sheet does not stay tucked in quite as well. It is on my to do list to get a piano hinge and make an extension I can flip out when parked and set up and flip back over when I have to put slide outs in. The mattress squishes up a little when both my slide outs are in but goes flat and right back into shape when they are put back out. Not only is it more comfortable but all my queen fitted sheets fit better!