'Bout the only way to fix it is to either replace the sheet or to grind all of the cracks out and all the way to the bottom of the cracks, and reglass, reapply the gelcoat or glass and paint, or some combo of those depending on the depth or extent of the damage.
I have something similar on one of my side walls on a slide (the wall that the wiper seals rub on). But replacing the sheet is easy to do on it. Just remove the trim and silicone and just a small piece, probably like 40"x76", so easy to handle....after you take the slide out.
Heres my cracks