We bought a Coleman PU many moons ago that was new but had been sitting on the dealer's lot for a couple years. After a few trips and some eventual rain, it was evident the canvas was dry-rotted and leaked like a sieve. I took it back to the dealer, and since it was classified as a new PU, I was able to get them to replace the canvas free of charge. If it were a used PU, then I would have been out of luck. Obviously we don't have to worry about canvas in our TTs, but there are other features that would be similarly treated depending on whether it is new or used.
Regarding used TTs, the big issue you are going to have is availability of specific makes and models that fit your needs. When I bought our new TT this past summer, I generally knew which floorplan I wanted and what I was willing to spend, and the salesman was able to show us several models and manufacturers that fir the bill. When I asked about used TTs, he said there weren't any that would meet my requirements. And from what I saw on the lot, used TTs that were 2-3 years old weren't exactly cheap. It seemed you needed to go back in the 5-10 year old range to start seeing a big difference in price between new and used.
But my recommendation would be to determine your requirements and needs (as opposed to wants), and then start looking through inventory, both new and used.