I apologize up front now since there are no before and after photos.
The 'little bit' of slack in the wheel well skirt was actually huge: To grab the center of the opening and tug yielded about five or six inches of 'pleat' slack. This is a huge strike against working alone when one pair of eyes can't see everything even when leaving enough time to try and think things through.
I do know the shell was stretched from the back curved C-channel very tightly, the battery locker cut-out on this trailer seems to have been a fulcrum point when the outriggers had failed or gone missing and stretched the heck out of everything, out-of-balance wheels, road strikes, tail dragging sure became additive to un-taut sheet metal forward of the locker.
I just kept repeating the mantra "what I'm here for is... melting (block) ice!" whilst burning 1-1/2 pounds of propane, and so far have melted fifteen pounds of ice doing heating and quenching. This is not something to try try unless you're able to replace the entire sheet, and have the interior, wiring and insulation stripped out. The mid-shell aluminum on this trailer always had a puffy appearance on that side, the sheet lifting between rivets.
I started from the belt-rivet line and worked aft taking out any 'oil-can' flex to the sheet, and cascading down to the un-riveted C-channel sheet edges. As I would reach the well opening and shrink the skirt edge it was apparent there was more slack farther away, each wave of bagel or slice-of-pizza' overlapping heated area only reduced the swollen effect by mild increments, so another round of shrinking, from outside to edge would get repeated.
With iron/steel the temperature differences achievable are greater so using a soaked towel works well, with aluminum you can't let it get too much hotter than seeing an energetic boil-off to water droplets from the last quench. Having the entire target surface wetted when starting to torch an area is key to observing where the heat is flowing.
Another point to make is after finishing a two or three-hour session and walking away it takes a few hours or overnight for everything to normalize while slack in distant rivets or shell rib movements gets drawn to a new baseline location. Literally applying heat on one side and see the far flap of the wheel well flex in-and-out three inches is a little bit spooky. I probably will be drilling some rivets today, and buck in filler and replacement rivets later, to ensure everything is anchored and water tight.
I have still fifteen more pounds of ice, I'll be up to four feet fore & aft of the wheel well shrinking metal to draw it up tighter. I get some photos this morning and see if I can show the results.
The days are short and the night is long and the stars go tumbling by.. . ~Airstream~