We found a 4" memory foam mattress online that was an exact replacement size. As i recall it was an odd size queen. Works great. We probably have the same one Steve and Linda have.
To try to extend the old mattress' life, we bought a topper, which made the total thickness impressive, but you kept rolling in and out of the trenches during the night. So, we ditched the old mattress and the topper in favor of the above. It gives us a little more headroom in sleeping cabover.
regards, as always, jefe
'01.5 Dodge 2500 4x4, CTD, Qcab, SB, NV5600, 241HD, 4.10's, Dana 70/TruTrac; Dana 80/ TruTrac, Spintec hub conversion, H.D. susp, 315/75R16's on 7.5" and 10" wide steel wheels, Vulcan big line, Warn M15K winch '98 Lance Lite 165s, 8' 6" X-cab, 200w Solar