Correct, bending in the vertical plane is what causes the cracking and not tensile or pulling.
Also correct is squaring the frame, which is what the cross member actually does and causes the stress to be shared across the frame ans not be focused in the bend.
When the bend is boxed and the cross member is welded to the inside of the box it also strengthens the area.
If the trailer "box" is mounted on a trailer frame that is a pure triangle back to the axle
this also removes the stress in the bend since it is not there.
The stress on the driver side is amplified by the door passthrough on the street side which has more flex than the other. This extra flexing (in the vertical) is why the driver's side tends to break first.
Right behind the cracks on the driver's side are the attach points where the sideways tubes under the door transfer the load to the front of the frame.
My guess is that many more are broken out, but harder to see since this is on the top of the part of the door tubes where it is hard to see with the floor in place.
This is a view of this ares in my modified Scamp
frame note the tubes coming under the added cross beam.
Here is the outside view. In the stock version there is just a piece of angle and a sheetmetal cross piece. The area where the angle was welded was completely broken out.
As you can guess a bit of weight
was added when the frame was extended and reinforced. I also added 1 1/2" square tubing to reinforce the floor all the way to the sides of the shell tying it into the structure of the overall support of the rig.
In the original Scamp
the only real structural attachment of the shell was where it passed over the frame in the front and back.
I also added a 1 1/2 X 3 tube under the area of the original bend.
All of this was to add the space for the front bath square floor pan.
Here are some of the added side floor supports and new steel wheel wells to accommodate the larger wheels and tires
The new 3/4 " exterior plywood epoxy glass covered on all sides is epoxy glassed to the shell all the way around the trailer making the structure a part of the frame instead of riding on OSB sheetmetal screwed to the frame and just stuck to the shell with no real structural strength added to either the frame or the shell.
Here is the rest of the reason for the extended frame.