When I bought the Lite*House, the seller pointed out a small leak next to the vent fan. She had used butyl tape to reseal it but the repair hadn't held. I decided to tighten it up.
My first attempt was close but I made some mistakes. My second attempt was more of that nasty butyl tape. It lasted until the first storm.
This is my third attempt and hopefully, this time I got it right. I pulled the fan, peeled back the insulation and carpet, and scraped, scrubbed and sanded the fiberglass until it was smooooooth. My cabinet making buddy traded me work for work. I'm gifting him some Macintosh consulting. He made me a pair of birch plywood trim rings that match the rest of the bright work in the trailer. One ring was epoxied to the inside of the roof using West System 610. That stuff is magic. I cleaned the fan flange using high test rubbing alcohol and installed a new gasket. The fan was screwed to the flange from the outside. A thick bead of Dicor lap sealant was run around the edge and over each of the many screws.
The second, smaller wooden trim ring is glued and screwed to the bigger ring.
We had a big storm this afternoon and everything stayed dry.