I had the same issue with both my '77 1300 and '79 4500. Like the vinyl insert in the belly band, with time, these shrink. I found out this is a type of spacer to keep the window protected within the frame; the glass is set in and adhered to the aluminum frame. The aluminum frame is set into the door with butyl tape as the other windows
I carefully pulled away about 1/2" from the window and drew a picture of the side view. Took another trip to my local RV store/service company (in Seattle, it's Evergreen RV) and bought a length from the service area. Pulled out the old/brittle/shrunken seal, pushed in the new and it looks beautiful! One of the simplest fixes on the trailer!