Clamps are the first thing that came to mind for me, also. I grabbed 2 of my pistol grip compression clamps from the garage and stepped outside real quick, opened the door of the trailer and they fit over the drip rail and into the inside of the trailer. Gave them a couple of squeezes and it looks like they were made for the job.
Material needed to do the job would be a center punch, drill with 1/8" bit, a handful of Olympic rivets, a good rivet gun, mineral spirits to clean off the old Vulcem and a tube of Vulcem or ParBond (never use silicon on aluminum). A rivet shaver would finish the job if you have one, $expensive$. A Dremel type rotary grinder can be used if you are patient, to smooth the rivet heads.
The 72 Tradewind has the Suicide mounted door. Has the door ever flung open while traveling? I would think this would have to happen a couple of times to pull loose like you describe, but I guess one good time at speeds could cause quite a bit of damage.
Also what is the condition of the floor at the entrance. If it has failed, then perhaps when stepping in and out of the trailer and stepping on the step and the bottom door frame, it could be pulling down due to lack of support at the bottom.
-"You want to make it two inches - or, if you're working in centimeters, make sure it's enough centimeters for two inches."-Red Green