Before I had my trailer raised the last time, I caught my plumbing on a rather bad dip going into a parking lot. I didn't realize it until I got home, but I left an entire section of plumbing somewhere. It also caused it to crack the fiberglass in a few places where the shower drain came through the floor. Repairing it underneath was a little messy, but it was easier than the other repairs I've made because I didn't have to add gel coat to make it pretty.
I roughed up and cleaned the area, then repaired it with several layers of fiberglass soaked cloth. That was several years ago and it has held up well. IMHO, the hard part of fiberglass repair is the gel coat on the areas that show. I have done that as well and it came out okay, but I sure couldn't tell anyone how to do it. It takes me forever
If yours is underneath as mine was, then you shouldn't have any problem. Other than fiberglassing my hair to the bricks I was laying on and ... well, lets just say that I recommend long sleeves and goggles when you're work overhead.