Roofs-Fiberglass vs. Rubber/TPO

I have a Winnie with a fiberglass roof and the issues with these are not the sheet of fiberglass, it is the air conditioner seal, penetrations and sag. The air conditioner just sits on a foam seal right on the fiberglass roof. The roof has a curve across it so the seal needs to make that up. Over time the water will get into this seal. The structural roof deck tends to sag from the weight of the air conditioner and water can pool around the a/c and leaks in. If the deck sags, it pulls the outboard edges of the roof in and away from the aluminum moldings (as you can see in the photos below). Fiberglass is rigid and does not stretch like rubber. When you take up an a/c unit to replace the seal, you need to be careful not to rip the fiberglass. It does get a little brittle over time. You have to keep an eye on the front and rear cap joint. A good flexible seal helps there. So it is not the sheet of fiberglass that is the issue, look at the other parts of the equation.