We used a memory foam topper on our last TT's mattress. It was OK but it shrank down so it was about 3" narrower than the mattress. It also liked to move around and try to slide off one side of the bed. So I don't know that I would recommend one.
The mattress in our new Northwood is pure junk. My wife and I (135 & 170#) have left permanent dents in it after only ten nights. I figure we'll flip it and get one more season out of it. Then we'll be in the market for a new mattress next year rather than just a topper.