I had a similar experience and it was the bottom horizontal extrusion that had lost its seal to the slide face. Water was seeping behind the extrusion and running along the bottom edge of the extrusion, around the corner, and into the camper. It took me a long time and many buckets of water to find it. After ruling out the seals (the rusty screws in the bottom of the extrusion gave me a clue), I kept assuming it was the window in the slide. Scratched my head so much I was almost bald by the time I realized the problem.
Water leaks can be very difficult at times.