Here are a couple of useful links:
If things are in relatively bad condition, it's best to remove the fixture (vent, etc.), clean up all of the old sealant, re-seal and re-mount. I think you'll find that the most commonly used Winnebago part number will cross-reference to Manus Bond 75-AM sealant.
If things look to be in relatively good shape, you can clean things up and use Eternabond tape to cover all of the mounting flanges, extending over the edges to the roof. Personally this is what I chose to do. You'll find some nay-sayers but most of us who've done this are happy with it.
Whatever you choose to do, remember that gooping on globs of sealant over old sealant isn't going to do the job. You're never going to see gobs of sealant on a well-kept sailboat. In fact, you're going to be hard pressed to see any sealant since it's all between the fittings and the deck. Even the screw heads are shiny metal, the sealant is under them and in the screw hole.
And, although many will disagree, despite it's popularity, Dicor is not formulated for fiberglass roofs and is only specified by Winnebago for the rubber roofs on it's towables. Will it work, probably, but why not use what Winnebago specifies for its fiberglass roofs?