Look close and see if it isn't pitting from the inside-out....  Those doors are filled with foam, water eventually leaks inside, soaking into the insulation, holding water against the bare aluminum inside the door causing it to "rust" oxidize from the inside out... Couple of fixes, replace skin and seal door to keep inside dry, or, replace with a new door.

http://www.pplmotorhomes.com/parts/rv-locks/rv-entry-doors.htm

You will need access to the inside of the panel to effect this maintenance. To stop the corrosion, you will have to apply an acid by scrubing the aluminum with a pad. When the acid has stopped reacting, the panel will then have to be generously flooded and rinsed with water. If there is any corrosive pitting, then it will have to be addressed with an epoxy filler.

If you are able to reach the inside panel to effect these actions, let me know and I'll furnish more detailed instructions.

well, I noticed my camper door doing the same thing.. it's a 1999 Featherlite...

I don't think there is any way I can access the panel interior. I'd love to just take off both sides and put a heavier alum on the outside and another panel on the inside... like the inside of the horse door.

Sad that the stuff reacts and the manufacturer didn't come up with a better solution.

Jbsny