*sigh* Just when I think I have found it, it doesn't pan out.
It does now appear to be coming in from the lower front of the window frame, between the frame fascia and the wall. What's strange is that spraying the hose right at amy of that area on the outside doesn't seem to trigger the leak. But spraying up above it does. Sometimes. It's very sporadic and thus far non-repeatable. At least I know where it seems to come out on the inside. Just need to digure out the in on the outside.
And now I suppose plumbing is definitely the wrong category here being that it was never the vent pipe.
It may be a gap in the butyl sealer under the flange of the outside window frame at the top. The water is running around the inner window frame and exiting on the inside. The quick and easy fix would be to clear caulk around the outside perimeter of the window frame.
I like Russ's idea of using masking tape.
This is common with the external frame windows and the age of our coaches. I used a
Polyurethane sealant rather than silicone. Masking is tedious but produces a superior result.
It's easy to think of water running downhill.
It's a lot tougher to consider capillary action where the surface tension of water pulls it into crevices and will actually go up and around.
i use the polyurethane as well. but.....very important to mask off both sides of where you are sealing. that stuff is nasty. i used pin stripping tape to go around the curves.
to
So just to close this out - thanks to everyone for their help and ideas.
It seems that my decision to not assign immediate importance to the roof vent cover was not to blame.
And once again, Richard was correct. I found at least 2 places around the window frame that were leaking, both leading to the front lower corner of the frame before coming out both on and behind the wall paper.
One place was literally right next to the point of ingress and you can visibly see a gap in the butyl rubber behind the outside of the frame. The other was less obvious - right near the top center of the window frame, along a seam in the aluminum bus shell. The shell was slightly pushed in at this spot, enough for water to get behind the frame.
After putting a bead of sealant all the way around, I sprayed it for quite a while yesterday with no new ingress. As soon as I can get it in to someone who can do the work, I plan to have the window frame pulled out and reseated with a new seal so that the thin bead is not the only thing between us and the rain.