Amazon has a wide selection of memory foam RV King Size mattresses also. We replaced our original mattress with a 72 x 80 Brentwood Gel 13" Memory Foam one and it made a huge difference! As for mattress pads or sheets, California King seems to work a little better than regular King for getting a better fit (I have a lightweight mattress pad on the RV Bed in Calif King), but since I have a King Bed at home and had extra sheets I just use those in the RV.
If you go the memory foam route, don't unpack the mattress until it's in the bedroom. They vacuum seal those things and boy do they expand!