I fixed my floppy lower door on my Boler
when we first got it (20 years ago).
My fix took about one hour and solved the problem.
I used stainless steel aircraft cable (1/16"), some small turnbuckels and 3 1/4" stainless bolts and nuts.
I used some fender washers and a grinder to make a teardrop shaped washer with a 1/8" hole to secure the aircraft cable. (this would have been better if the fender washers were stainless steel, but, I couldn't find what I needed when I needed them so... you will see some rust on the washers).
In my case, I was able to use one of the existing fasteners from the door latch and only used 3 1/4" bolts. These 3 1/4" bolts are the only thing visible from the exterior. Use double nuts on the bolts and DO NOT overtighten the bolts, this will crush the door. Snug the bolts up and lock them in place with the double nuts.
The turnbuckels allow for adjusting the tension of the cables and this draws the lower section of the door against the camper and allows the door to seal.
The photos should be self explanitory.