I find it really hard to believe that water is getting between the panel(steel) and paint. I don't see how that is really possible? anyone else?

 

please post pics

 

My thoughts on the deal are that water gets in the little black vent in the door jam area, just above the cab corner, and over the years it allows the cancer to just take over.

If your panel still feels nice and sturdy, I would suggest have that area looked at very closely to determine why the bubble of water is there. This area is known to rust out on dodges, as well as the bottom of the doors and around the wheel well lips.