Have you considered a Memory Foam mattress?
Before Memory foam was a thing we had a custom foam mattress made for our first TT. Was comfortable and the foam unlike a spring/air/water mattress does not transmit much if any movement.
Our current TT we had a custom mattress made which includes a 2" layer of Memory foam as the top layer.. We call it the Marshmallow..
If you go with Memory foam, make sure to get one with the cooling gel, Memory foam doesn't breath as well as standard foam and sometimes it gets too hot in the summer if you don't have air conditioning..