This is one area where you have to "over maintain". When new I redid all the seams on our Lexi (fiberglass roof) with Dicor. All, vents, side rails, antenna, front and rear cap where they meet roof.
Over lapped the front and rear cap seams with 4" wide Eternabond, bringing it all the way down to the side rails. Eternabond in each corner, where water pools, as other posted.
Never had a leak in over 3 years.
That being said, I still get up on the roof every 2 months or so to clean and check caulk. Even Dicor, as good as it is, will still develope cracks over time that need to be cleaned and filled.
A little time and effort spent on a regular basis can save a whole bunch of problems from occurring down the road.
Steve & Cheryl + Charlie & Casey, our furry kids.
2012 Forrest River Lexington 283ts
Toad, 2013 Subaru XV Crosstrek