I posted an exterior door install a few weeks ago. Aliner folk were not much help. I felt better building a little frame for structure after cutting a big hole in the side of the camper. I also raised the hole as high as it would go, to leave some of the side panel below the hole. Leaving that little bit of panel on the bottom, gives that side a little more structural integrity. When you cut the hole, there is the outer and inner skin, and the exposed foam between them. I made sure I covered the exposed foam with caulk, and finger worked it in, to minimize water intrusion. Put a good bead of caulk on door frame and bed it in the hole. Clamp the frame in the camper side for 4 hours or so. After 24 hours for a full cure, it should be good to go. Since I've been a mechanic for years, I put screws through the frame and into wood frame I built around the door.