First, seal the window frame and top of the door to stop the water leak that caused it. The entrance door and compartment doors on certain year Camplites are made by Challenger doors and had lauan plywood in them. That gets wet from leaks and corrodes the aluminum skin. The typical leak is at the bottom of the outside black plastic trim around the door window. There is a small joint there that lets water inside the door. I used clear silicone on the crack after a good cleaning. Once the door no longer leaks it will take weeks, if ever, to dry out even in the hot sun. Once dry, a light sanding and spray painting the door is one possibility. Others on here have replaced the door, but that's expensive. It is a standard size RV door, so do some on-line shopping for a good price.
Edit; on my compartment doors, removing the blisters left holes in the aluminum skin, so I replaced all 3 doors. .
Edit; on my compartment doors, removing the blisters left holes in the aluminum skin, so I replaced all 3 doors. .