I'm a big fan of Eternabond tape (it's been around for decades) that I've used on both of my RVs. One with a rubber roof and my newest with the fiberglass roof. I use 4" Ebond on the end cap seams and 2" Ebond along the edges, full length.
After reading many postings by owners of various roof issues over the last 15 years, I went ahead and covered the seams with Ebond so I don't have to mess with it very often. Getting on up in years and sometimes get dizzy climbing up on the roof so that's a factor. Had it checked at the factory a couple months ago and it's still in good condition. No needed maintenance.
Here's my blog entry about what I do:
Fiberglass Roof maintenance...