Depending on your style of TT you want to pay special attention to where the roof transitions to the front and back wall. My Jayco has large rounded edges where the front and back walls meet the rubber roof. I have constant problems keeping it sealed. When the TT was two years old I was constantly putting Dicor lap sealant along the rear transition. The Dicor was constantly separating along the transition.
This summer I once again had a leak. I cleaned off the Dicor sealant and resealed the transition with Dicor tape and Dicor lap sealant.
I also had a recall on the decorative trim piece along the rear top of the trailer which I suspect lead to the leaks because it wasn't repaired correctly under the recall . I'm not impressed by Jayco's design of the 2009 Jayco Eagle roof transitions. It was about looks and not function.