I've done a few tow trucks that had fiberglass shells. Not sure if they are made of the same materials or not. And I just treated them like a Large SUV + $50 for my pricing. Wasn't sure how tocharge either but I was fine with it.
I have two that I do regularly. I charge by the hour for about everything these days. The hardest part about the two I do is polishing all of the chrome and aluminum, and dealing with bug gut build up.