When it is dry and door is closed gently spray water on the side of the rig and let it run down then open the door and look for where it may be dripping. Advance the test while getting a little more aggressive. You might even put a small piece of Kleenex or something at the pilot area to determine when it happens. Also consider if the rain may be hitting it at more of an angle. From there you may be able to make some sort of small baffle or deflector to shield it. When I installed my WH I noticed how irregular the edge of the sheet metal flange was when caulking it and that may contribute to it dripping in at just the wrong spot.