Over the years types of sealants have always been confusing to me. What to use where. A few years ago I stumbled upon this article about sealants on boats. I think much of what works on a boat works on RVs. Plus this article has a link to excellent butyl tape and a sidebar on how to use butyl tape. It has helped me a lot and so I am passing it on. It a great article and a must read for anyone contemplating rehabbing a RV. I have purchased and used the Butyl Tape from the link in the article and I can attest to its quality.
http://www.boatus.com/magazine/2013/April/how-to-pick-the-right-sealant.asp