Billy....Typically, the window is sealed with putty or butyl tape under the frame to seal it to the side of the coach. Then a bead of silicone is run along the top of the window exterior frame, and then down both sides about 2"-3". This keeps water from pooling on the top of the frame.
First, tighten the screws on the inside fo the window frame, snugging the window tight against the exterior. Then check that the silicone bead is in place.
If this doesn't resolve your issues, you may be leaking at the top of the slide, running inside the wall and then out the window frame. You'll need to get on a ladder and insure the top of the slide, probably near the edge where the slide topper doesn't protect it, and check for places where the water can make entry.
If the window is properly sealed, you could test the theory of a leak at the top of the slide by taping some plastic over the edge of the slide to the roof. Then see if you still have a leak.