WELL.....(deep subject)...so I got the stud welder out - no studs, just that hot tip. It actually comes with a rounded tip just for this. I located 3 epicenters of the oil canning - all about equal distance from left to right. FWIW, I'm about 80% certain now this was caused by the wind in the interior light hole, so note to self: ever do this again, a piece of duct tape is your friend.
I started on the far right (pass) side. Ground off a 12"x18" area of paint where the flexing was. Wiped it down and got a dry cloth and then a cold wet cloth ready. The rounded edge on the side (6" in) was still rock solid. I started heating a tacked area - maybe 3-4 at a time and spread out. Waited 2-3 sec and stuck the cold wet rag on it. You could see the tacked area drawn down afterwards. Maybe 50-75 or so later, the area was rock solid. Maybe better than before. Its ever-so-slightly flatter (I think), which will work well with a thin layer of filler.
Moved to the middle area and continued on. This was the worst as it was the (IMO) the source of the whole issue. It was like a water bed. I continued on a few tacks at a time...about 50-75, the whole area was really firming up. Whats more is, the far left side was also firming up and I never touched it. There is still some 'softness' around where the cab lights are going, but that area is contoured and holds its shape just fine.
I spent about 1.5 - 2 hrs on it. Its getting ready to rain so I'll mess with it another day, but UFB.....I would have never believed this had I not seen it. I like my stud welder a whole lot more now too.
So thanks everyone - you fellas are tops - I owe you a beer.