I'm not sure what happened - but I suspect that at some point, someone backed into a rock or tree with the jack, and broke the frame. Of course, they followed this up with a crap-tastic repair job: basically they took a couple pieces of flat steel strap and a hand full of sheet metal screws, and you can probably figure the rest out.
That corner was so full of holes that it was completely unusable. In a stroke of luck, I discovered that I didn't have to cut the corner uprights out. Nope, all I had to do was grab them with two fingers and pull. Yay.
Now, I don't know how many of you weld aluminum - I never have. Well, at that point in the project, I never had. Anyway, FWC uses .040" wall aluminum tube for the frames - at least they did in 1991. That is too thin we weld with a stick welder, or even a MIG. You burn through about 1/2,000 of a second after you strike an arc. Even the guys at Miller (who actually know what the heck they are doing) will tell you to not waste your time. Apparently they didn't fully understand how hard headed I can be.
It took some trial and error (and a lot of scrap), but managed to weld new corner uprights in there with a MIG. The trick was to go for a lot of surface area, putting as much of the heat into the new (thicker) uprights, and letting the puddle run into the thin tube - breaking the arc as soon as the puddle reached the old piece. Using scab plates, I increased the surface area substantially, in an effort to offset the lack of penetration I was getting. The welds weren't that pretty, but they survived the hammer test. Honestly - I wouldn't recommend this as it was a huge pain. If I ever do it again - I'll take the frame down to someone with a TIG, and pay them to do it right.
Anyway, since some of the siding didn't fare so well being pulled back - and there were a few holes that needed to be filled anyway, I decided to replace all of it. So, off it came, followed by the insulation.
Looking at the other corners, it appears that there have been several sets of jacks installed - leaving behind too many holes to reliably mount a new set of jacks. I contemplated cutting the remaining 3 corners out and replacing all the uprights - but without a TIG, it was too much of a pain. Since the structural integrity was there, all I needed was a fresh piece of aluminum to screw the jack brackets to. In the end, I used a piece of .065" thick, 2" wide, aluminum angle. Using an aluminum epoxy, I basically glued the angle to the existing uprights. I managed about 48 square inches of contact per corner. The epoxy has a shear strength of 2,600 pounds per inch - so it isn't going anywhere. To avoid over-stiffening the frame (resulting in fatigue at the joints), the angle was only attached to the vertical pieces in the corners - the upper and lower horizontal pieces are not attached to the new angle. The result is a fresh surface to attach the jacks to, without interfering with the frames ability to flex at the joints.
Since I had the welding gear out, I went ahead and moved the propane tank enclosure out of the 'kitchen', to the lower left corner. I added one horizontal piece of tube to frame in the top. It's a tight fit between the exterior wall and the door frame - but it fits. It's going to take some modification of the access hatch down the road - but it frees up a ton of space under the counter top. Essentially, I'm trading bulk storage for cabinet space - an exchange well worth the costs, to me anyway.