The problem with repairing rusted frames is that it is very difficult to weld to them without blowing out the metal. These early frames were pretty thin to start with anyway.
I didn't replace my entire frame, but I did replace the front 1/4 - 1/3.
I added a lot of extra bracing and used thicker wall rectangular tubing and added a full two tubing sections across the front.
These frames are wont to crack in the left (driver's) side curve and where you can't easily see it in front of the door opening.
This is caused by the flexing of the frame under the door opening.
I think that all of these frames are prone to cracking in this area for the same reason.
If you look carefully you can see the effect of corrosion and cracking, Repaired?
Here is how I welded a second tube under the new frame back to the doubled stock frame.
This is what I did on the door side. The bracing and the tub was cracked through here.
This is the weak area which causes the cracking on the other side (In my opinion)
This pictures shows the extended frame to allow for adding the front middle bath.
You can see the reinforcement added at the bottom of the frame to the new carry through beams.
Of course this added a good bit of
weight to the trailer and the added bracing tubes from the frame to the sides to carry the new 3/4 plywood that was bonded to the shell did too.
However now the shell is bonded to structural steel all the way around. Before the loads were basically in the four areas where the frame passed under the shell and the sides just basically floated and added no strength or stiffness.
Originally the OSB was just screwed to the top of the frame and was cantilevered over to the shell and the shell actually supported the sides of the floor.
Heavier, but maybe better?
Here is a picture of the new steel wheel wells and the added 1 1/2" square tube bracing.
Here is a good part of the reason for the extended frame. The bathroom pan sits in the new extended frame. You can see the added bracing for the area in front of the door.
All of the above is not necessarily to show how it can be done, but rather to show how one change leads to three more.
The modifications and repairs lead inevitably to more.
The more you look the more you find that needs to be fixed.
If you are not looking for a project then find a good trailer.
If it smells musty then carefully check the floor for rot and the frame for rust.
One thing leads to another............