Not all silcones are created equal.
yes, many of the hardware store silcones aren't all that great for RV use. Many are aimed at indoor use.
However, 3M and others do make silcone caulk that work extremely well for Boat and RV applications on metal and fiberglass, Tad bit expensive, but they do work and do hold up. Places like west marine etc. have lots to choose from.
Trempro (sp?) is another brand that has silicone formulations made for exterior use on RV and boat applications on metal and fiberglass.
Proflex and some other non silicones work well also, but the don't last "forever" either. I've found the high quality 3M silicone or Proflex lasts about the same time before needing to be redone.
I agree on epdm roofs, no silicone, that's what dicor is for
