Sorry to hear about your problem.
There could be several reason a crack appears but you would only be guessing until you have a professional body shop that does fiberglass work inspects it.
You stated the crack is at the back of the motorhome. Do you mean on the rear cap of the motorhome or on one of the sides at the back?
If this is a know issue at Winnebago they may be able to assist on explaining how to repair it and preventing a re-occurrence of the crack. I doubt they will assist financially.
Good luck and I hope it is an inexpensive repair.
Paul
