I've got a Tepui Ayer, which isn't a large RTT. I found a double-sized sleeping bag that fits it that I was going to use as added padding, although it wasn't very thick. So I recently found a 1-1/2" gel topper to go inside the bag. Unfortunately it's a bit too thick to leave inside the tent and close it up, but the hole thing folds in half and rolls up well enough to place inside a canvas bag. The bag then goes inside my trailer. It takes up room (same as my old ground tent), but that should really improve the comfort level.
It would be nice if the RTT allowed for a thicker mattress to begin with... The entire reason I moved up from a traditional tent to an RTT was to get off the ground as I'm starting to age. (How was it that 20 years ago I could sleep on the ground with just a basic sleeping bag but that's not the case anymore?!)