I highly recommend using Ames Maximum Stretch paint and their seam tape. Used it on my bus because I was also concerned that it could leak. This stuff is designed to flex and is made for metal roofs. It is a strange material but after going through a brutal snowy and wet winter it worked as expected. It also cools the bus down a lot in the summer. I have pictures in my album and more details in my build thread.
The paint and seam tape total was like 200 and I hit all of the seams on the roof, around the vents, and the edge seams as well.