I ended up finding my butyl tape at the RV dealer. The issue you'll have is it comes in a larger roll. Not huge, but more than you will probably need for now. For an alternative to dicor sealant, I used OSI Quad Sealant which is available at Home Depot and it's been awesome so far. Obviously, for the roof you want a lap sealant like Dicor's lap sealant but, for what you are talking about the OSI caulk would work if you don't want to find Dicor or Proflex brand caulk for RV's.
For the screws, I'd put a dab of caulk either on the screw or if you cut the tip fine enough in the hole when you run the screw in, it will seal iit up extra too. Just don't use too much as obviously it will probably run over and make a mess.
2012 Keystone Sprinter
Still sticking around for the good people and advice.