Leak

Driving home on interstate the other day I ran into a downpour. After awhile a couple of drops of water came down , looks to be from where the fiberglass roof over cap meets the metal vehicle roof, 09 fleetwood class c on dodge chassis. It appears to me that the seal there is a gasket rather than sealant, does show some signs of age, minor cracking. I have read some threads, some say to use dicor, never silicone. I also read one guys dissertation that dicor is marketing hype, only good for laps on rubber roofs, he says use quality silicone. This location is obviously a tight spot that must withstand driving rain at 70 mph. I don’t see how it would be possible to get in there with any of the tapes. Anyone have experience sealing this area on a class c, with recommendations for what actually works best? Any help appreciated.