You probably need to closely inspect the entire Seal against the Fiberglass and the Glass to ensure that there is Sealer continously against the glass and the Fiberglass. The problem with our windshields is, that the glass is heavy and moves within the Seal and the outside of the Seal moves against the fiberglass as well. If there was not enough Sealer used when the windshield was installed it may have dried out, and or been displaced by the vibration going down the road. If you can't get the water stopped, you may need to have the windshield removed by a "GOOD Glass Shop," if you can find one. It doesn't take them long to pull the glass, cleanup the residue, and reinstall the glass. You should be ready to go after that. Hopefully, you will be able to find the void that is causing your problem w/o having to resort to glass removal. Good luck.
2002 Monaco Diplomat Triple Slide PST
2011 Buick Enclave(AWD);
1970 Home built 11.5 ft Slide-in Cabover