Winnebago publishes a sealant guide for your coach. Give Winnebago Service Assistance a call at 800 537-1885.
I have a fiberglass roof and use Dicor self-leveling sealant on all the top mount fixtures on my coach. I sticks fine to my fiberglass. If silicone sealant was ever used on the skylight that could cause you a problem. I would scrape all the old sealant off, clean the area with denatured alcohol and then try the Dicor again. You almost can't use too much of the stuff. Spread it up the side of the skylight and out well past the edge of the previous sealant line. MAKE SURE you are using the self-leveling stuff. The two Dicor products are easily confused.
Hope that helps.
Note that I DO NOT use DICOR on most of the other joints on the coach. Winnebago uses a couple of different types of sealants and it is important to use the same type when you reseal.
Randy and Tina & pups; Cinnamon, Cookie & Coco
2010 Winnebago Journey 34Y
Freightliner / Cummins 340hp ISB, 2014 Ford Focus