I have had 3 TTs.
The first one was a 78 Prowler. It had a pretty heavy looking frame with some of the worst welds I have ever seen. No issues though. No idea who made the frame.
The second one was a 2001 Fleetwood Wilderness.
It had a beefy frame as well, and what I would call average welds.. I did have some stress cracks, and torn spring Hangars, which I welded up myself.
I think it was a Dexter frame. I know the axles were.
My present TT has a Lippert frame. It is the strongest one yet. The welds are first rate, and the axles are really nice as well. Everything powder coated to boot.
So based on what I have seen... I prefer a Lippert frame.