Since no else has said it, an Aerobed works well, too, if you can sleep on an airbed. We never did a single night on the stock block o' foam - we're keeping it minty for if/ when we ever sell/ trade the TC.
An airbed puts a big chunk of air between your body and the cabover floor - this insulation keeps us warm in bed in an otherwise cold camper. It's a familiar/ comfortable sleeping arrangement for us. It's mostly air, so likely much lighter than memory foam (we have a Temperpedic in the house, it weighs a ton). You do need a shot of electricity to top it off from time to time. If we needed countertop, we could deflate it and flip it in half, giving us half the cabover.
We're Aerobed people, tho: we've had several, for several years, and taken them everywhere: visiting relatives, tent camping, drive-in movies/ camping in our Honda Element, etc.