Good manufacturing should prevent cracking of Fiberglass skins. Are the cracks in the corners? Do your windows have a radius in each corner? It's critical to not have a "sharp" inside corner otherwise the FRP will crack, no matter if you go off road or not.
To stop the crack from spreading: Drill a small hole (1/8 diameter) at the end of the crack. That will prevent it from getting bigger. Second, you have to put a cover plate on top. Such as 1/8 fiberglass or Aluminum skin. That coverplate should overlap by a couple of inches. You can use sikka to fasten that plate. Make sure everything is sealed at the end .