The best material I have found to seal seams on the roof of any RV is Eternabond Tape.
http://www.bestmaterials.com/Eternabond.aspxI installed it on the roof of my MH about two years ago and have not had any problems since then. I used it around all vents, skylight, etc. Last Fall I used it on the seams on the roof of my enclosed car trailer.It will not crack like Dicor sealant or any other sealer will over the course of time. The key is to clean the area very good and use a small roller to apply. You can find instructions at the site I posted.