A Full Queen will work in the cab-over (I prefer the romantic term "Loft") and allows use of standard sheets. A little hard to make but you can pick what you want. There's no need for the extra length (width of the coach body) that renders OEM cushions around or over 8-ft long.
Needless to say, doing this will make the Loft a permanent setup. For us it was worth it.
We replaced a full queen in the house and stuffed the old one into the Loft. it was an Innerspring mattress, with the hard wire around the edges. If you're buying one, DON'T get that. Get a Foam Mattress. It took three of us to cram that innerspring in place, and it stayed there for the rest of the time we had the coach. Stayed there when we sold it.
EDIT: Many places will "sell" you an RV mattress but it turns out to be an "order" and I don't want to commit to one that we can't try. Maybe in a different size, but we at least want to try it.
If God's Your Co-Pilot Move Over, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100 218" WB