Hey-o!
So, like all my posts, this is purely my opinion and by no means a professional one…
But honestly, the thought of mounting just in the fiberglass scares the crap out of me. I just mounted my solar panels to my metal roof, and even that scares the crap out of me! Because I just used the Z brackets and bolts as well.
But, in the front and rear, I couldn’t bolt through (for various reasons) and so ended up just having to screw them in.
Again, purely my opinion here, but maybe try some Steel Flats in the interior underside or something? Just anything to give it more hold than a fiberglass roof. You might be fine 95% of the time, but I was just watching a video where a guy mounted his solar with FAR more hardware than I did mine, and a strong storm rolled through and blew one of his off while on the highway.
So if he had more hardware up there than me, and I have more than you… You see what I’m getting at there. 
That’s my guy I just finished up. 