I had the same problem on my 21 Bigfoot
. I first removed the glass and it was apparent that water had been getting into the door for some time. I removed the door from the RV and laid it down on a piece of plywood supported by sawhorses. Remove all the screws around the perimeter and the edge extrusions will come right off. Be careful not to bend them. My door was filled by solid foam panels with a softwood perimeter of 1/2 inch X 1 1/2 inch strips. They had rotted from the years of moisture from the leaking glass. I went to Lowes hardware store and they had red oak cut to the correct dimensions so it was easy. I used a brad nailer with 1 inch stainless steel brads at the corners of the wood and reinstalled the inner fiberglass
panel and then the perimeter aluminum extrusions, being particularly careful about attention to square. After getting the frame square using a carpenter square, measure diagonally both ways to make sure it's right. If there is a slight discrepancy, make sure it is in the "too high" direction and not already in the sag direction. Once you are happy with the squaring, go ahead and screw it all back together. Mount it back on the RV and seal up the glass. I added an extra piece of Starboard to help keep the door from sagging and a place to mount a door holder. Good luck.