When I owned my 2000 Coleman Westlake, I had the same issue. I eventually found the leaks were where the bag and track were fastened to the roof. I took the whole thing off, and re-installed with new bag, caulk, and epoxy (where screw holes were stripped and/or had cracks). My Westlake had the one piece Royalex roof, so keeping/fixing water leaks through the outer skin was imperative.
then: 2000 Coleman Westlake
now: 2014 Rockwood A122 A-frame