The lifters are held in place by two prongs that come out of the vent frame. Stick a screwdriver in there and pry one of these prongs apart from the lifter and it will come right out. Once both ends of the Astrodome are loose, you can maneuver it around and bring it in through the vent opening. with the Astrodome off, you can service it and can access the top of the vent opening rim from inside the trailer while standing on a step stool.
Hope this helps. Good luck.
4CU 2699 / AIR 10 / TAC AZ-1
I'm haunted by aluminum.
Charter Member of the 4 Corners Unit.