Whatever the origin it means water is getting behind the gasket. It’ll probably show up inside the windshield as well. Had the problem on my last coach. I used a windshield sealer from the auto parts store. Comes in a tube and is like really runny silicone. I applied it to the outside edge of the gasket (on the outside of the windshield) on the top and both sides. Also to the sides and bottom inside edge of the gasket (also outside, of course). The stuff flows easily and goes where the water goes before setting up. It worked great and is invisible when you’re done. Took a couple of applications along the bottom inside edge in my case. It’s worth a try.
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