I tried the memory foam (2" thick) It worked OK, but really sucked up the moisture, and took up enough space that I had to remove the bedding and pillows to close the RTT.
I tried an air mattress both under and on top of the RTT mattress.
On top was a no go-got way to cold when the temps dropped. Under worked OK, but as mentioned, kind of bouncy.
I'm a disabled Vet, with back and hip injuries. I tend to toss/turn/flop/roll around quite a bit at night. Much to the dismay of my wife while using the air mattress.
Final solution? Picked up 2 self inflating back packing mattresses, one for each side.
Depending on how I feel, I can adjust the self inflating mattress to different levels simply by opening the cap.
With these slid under the RTT mattress, the set up has been working great.
The best part is, I can still close the RTT with the bedding inside.