Mine got wet because rain was getting in behind the lower trim strip (about 6" wide) on the outside wall of the slide. Rain runs down the wall; gets through the slightest gap in the caulking of the trim strip; and then "wicks" throughout the slide framing.
We chased possible leaks at the roof line and seams for 4 months before we happened (using dye) to find the entry point.
I sealed all of the trim and corner seams on all three slides with an elastomeric polymer paint designed for use on the roofs of mobile homes and available at Home Depot. Not a drop of water in the year since we put it on.