[jsb5717]

If you pulled the window out, could you see where the water was coming in? Was it wet at the bottom only or also around and on the top?

It's possible that the window isn't/wasn't installed correctly so that water is coming in around it. Also possible that there are in fact void(s) in the sill letting water into the wall below. Also possible that it's not the window at all but coming from the roof.

With the window out of the wall you could put some water in the track and see if it leaks through anywhere.

Also with the window out you can tape plastic over the opening to keep water out of the rig and watch when it's raining to see if there is any water moving into the opening from above.