Silicone is the worst thing to use to seal the exterior of an RV. It will not seal properly nor will it offer a permanent solution. Silicone will cause a leak and then the subsequent damage.
Nearly 40 years in the window / siding / solarium / patio cover business and I have made a ton of money scrapping products sealed with silicone and replacing them entirely due to rot issues from water leaks, all caused by lack of silicone adhesion.
I have installed many RV roof vents, Fantastic Fans and other things and found the best solution for sealing to be Dicor self leveling caulking. I have tried many different products such as butyl, elastomeric, one part polyurethane and others, but found the Dicor to have the best results.
Make sure the surfaces are thoroughly cleaned first and you won't have any problems.
