We bought a used '05 23B and absolutely fell in love with it. After purchasing it I started reading about all kinds of issues with the TPO roof and sure enough, within two months we had a leak and delamination on the driver side.
The dealer is splitting the cost of the repair with us and is installing an EPDM roof instead. I hope this works.
My advice is to buy new if you can. I have a friend who also bought a used TT and he had a roof leak as well. You have to worry not only about the leak but also about if you are going to have delamination, black mold in the wall, etc.
I would research the roof very, very well on whatever it is you are considering on purchasing - there are some very good videos on YouTube that point out what you should be looking for (as far as detecting a leak or previous leak damage) when inspecting a camper.