I used interlocking foam flooring (
like this) stuck to the floor with double sided carpet tape. It's helped a quite a bit, and added enough padding that the wife is happier - and the extra insulation is very nice on chillier nights.
It was about 40 bucks on Amazon. The mats sold by the RTT vendors may work better, and I don't know by how much - I tried this as an experiment and have been pretty happy with the results.
To be clear, I still get some condensation on the mattress - but it is less than it was and often not at all. The more humid the air, the worse it is, naturally. To a certain extent, I think moisture buildup will be unavoidable and so best practice is just to air out the tent periodically anyway.
Remember, you can't go too thick. A 1/2" pad is 1" thick when the tent is closed and there isn't that much space with bedding and such to start with.