First thing I would do is decide if you want to brace for a roof top air conditioner because now is the time to do it. Then I would clean the inside and lightly sand the surface. Figure out where I need the braceing and prop the roof up with a piece of thin 3/16" plywood at the roof and a pole to the floor where ever needed. The thin plywood would spread the load and conform to the the roof curve. then to put in reinforcing I would get some 1/2" round closed cell foam backer rod and hot melt glue that foam rod to the roof to create the form for an insulated rib accross the roof. Now I would wet out some 6 to 9 oz
fiberglass tape maybe 6" wide with epoxy and adheare that to the roof over the foam rod the whole length. Form this tape around the rod and flat against the roof I would bring this new
fiberglass accross the whole width of the trailer and bring it at least half way down the radius in the corners. This will create a stiff beam that is only slightly higher than 1/2" I would use a couple of layers of fiberglass. When you redo the inside your first layer of reflextic insulation you can glue between any ribs you have made and then the second layer of reflectic glued over everything to help hide the hump. When you cover with the rat fir things should come out fairly smooth. Thats the way I would approach this problem.