6 inch thick
Home RV 6-inch Reversible RV Short Queen-size Foam Mattress | Overstock.com
8 inch thick
8-inch Queen-size Accu-Gold Memory Foam RV Component Sleep System | Overstock.com
Thing is you don't want just memory foam, as it does not give much cushion. You want some regular foam, then memory foam. both of the above is for queen, has foundation foam and GOOD memory foam. The Wall-mart memory foam will not be very satisfactory, I know I wasted my money on it already!