The ones I have seen are actually attached to the vertical part of the vent and not through the roof. So you just have to make sure no rain gets past the outside. Caulking underneath doesn't deteriorate nearly as fast as the caulk in the sun. I don't worry about it.
Martin & Cheryl
40 Foot 2009 Damon Tuscany 4076
Cummins 360, Hyundai Santa Fe
Full timing since 2010.