I have done fiberglass repair on a holding tank 4 rigs ago. Its was a crack close to the exit fitting. For me it work very well however as I was working against gravity very messy.
I first cleaned the area then drilled a small hole at each end of the crack. That will stop the crack from getting any larger. Than came the fiberglass cloth, hardener etc.
It held very well for as long as I had the coach. There are other products out there now that may do as good if not better job. I am sure some folks will help out with that.
Good Luck, safe travels