Depending upon placement of exterior compartments, it may be no more expensive to simply reconfigure the bedroom area to accept either an RV Queen or standard Queen size bed. My suspicion is that it would be far easier to make an "island" type bed than one the spreads across the entire rear width of the coach. With a more permanent bed arrangement, you also maintain the accessible storage behind the framed in platform.
Good luck with your research!
P.S.: When I replaced the center twin mattresses in my '64 Overlander, I went with Lebeda
custom mattresses and the up-charge for customization was only $75.00 over the cost of the selected standard mattress. I have had mine for over nine years now and they are still absolutely fantastic!