Frames and axels

It's the axle imposing stress on the frame at that point. Well understood effect of torsion axles. You can see a finite element analysis showing the high stress location here:

I have heard of the cracks on the outer and inner walls from several people. The cracks predominately happen over the door, in the top left corner. Although your case may be different, It's possible the cracking has nothing to do with the axle.  If you look at the crack, it might be apparent that it was caused by "upward" force. The culprit being the stabilizer jacks. The door opening in the rear leaves a very thin strip between the door, and the rear of the camper. The stabilizer is able to put quite a bit of force lifting the back end of the camper with (for lack of a better term) a "hinging" effect. The rear corner being supported only at the top of the door. I noticed that on mine, the rear passenger area was easy to actually lift that corner with minimal effort. There was plenty of support for downward force, but virtually none for upward force. 

+1. In the meantime you can import foreign manufactured  vehicles if they're over 25 years old without them meeting federal standards. Beware your state requirements though. The Japanese ones I'm looking at are very practically designed, fiberglass coach work,  high on quality and low on unnecessary frills. They also tend to be well maintained and have low odometer readings. Very robust 4wd diesel truck chassis, don't think bent frames and axles are a problem. But they won't win any races for sure. Don't know about importing towables. 

Well, look at it this way. Upgrading leaf springs is cheap and simple and they're easy on the trailer frame. Often you can just add a leaf to an existing spring, not sure if that's possible on Lippert springs. If  that's all that's needed to get an inexpensive trailer to survive reasonable usage then I'd go for it. But unfortunately you still have the cabin integrity issues (water leaks and joints coming apart)...

   I am in the process of replacing the leaf springs on our Vista 19BFD the factory put 2,000 Lb springs on the axle with a 4,000 lb axle. If you check their website and look at the specifications of a 19BFD the 4,000 Lb leaf springs will not be strong enough and that's why I am replacing them. Broken leaf springs on their Vista and Vintage cruiser trailers were mentioned on Facebook owners forum and upgrading their leaf springs. I am replacing them with 2,500 lb leaf springs the factory 2,000 lb leaf springs were nearly flat from not being able to support the weight within the factory specifications. My experience with owning two FR campers and one Gulf Stream camper they are built with minimum quality and my own opinion they are built for looks not reliability. I agree with offgrid buy a used camper and do not buy new and hopefully the previous owner will have made most of the necessary repairs and upgrades.