Pictures would be worth alot in helping you get a solution
I am no body man but I have done my time working with fiberglass on boats.
Make sure you use Epoxy Resin if you do not already know this - The west system pumps make it very easy for the novice person to mix the hardner and resin together.
is it fiberglass with a rubber roof?
if you can get to the inside corner you can make a support piece or a frame so you can shape the fiberglass mat to match the existing surface - you will probably have to lay fiberglass matting down for starters.
hopefully someone with camper repairs jumps in here cause I am jst throwing ideas out to you to possibly give you some sort of an idea on an easy fix
2011 Rockwood Mini Lite 2306 - Matt
2004 Dodge Ram 1500 Qaud Cab 4x4 with 35" tires and 3.92 gears, Dynatech Longtube Headers & Fabtech 6" lift + Superchips with shift Tuning
Nights Camped 2011 (37) - 2012 (16)-2013 (2) -2014 (5) -2015(0)
USMC 91 - 95 - F/A-18 Aviation Ordnance