Whatever is used for the frame, it should be structurally sound. Most manufacturers use rectangular tubing or structural channel. Some use cheaper "C" channel or lightweight manufactured beams. I am surprised that some of the manufacturers haven't been fined by the NHTSA for building TT that cannot withstand the normal street usage. I've seen pictures of 5th Wheels where the front end is about to collapse.
Even Airstream had issues a few years ago with their 22' models.
There is no excuse to build a trailer with a weak frame.
