I'll relay a story about a friends camper that "broke" one day returning from Newnes campground, which is by no means offroad, dirt road at best.
Camper was a semi offroad made by a very well known Australian company that has /had a good name in offroad campers.
Suspension hangers tore the chassis away from where they were welded. Semi offroad you would presume the chassis was thinner than the full offroad model ??
Point being, no matter how well a trailer looks, something as simple as having the chassis strong enough is maybe overlooked ??
As previously mentioned, 50 x 50mm chassis seems OK, but is there different thickness steel in the same size ?
On my small van, the hangers are welded to a fair bit longer piece of angle iron that is then welded to the chassis in lots of different spots, so distributing the load over a greater area .
Maybe one area a good trailer maker already has lots of experience in ??