Had the same problem on our 2012 Lexington.
The roof material is not a true"fiberglass", at least on the Lexi. I was informed it's a type of fiberglass/plastic composite. And, it's only about 1/16--1/8 inch thick.
FR did not want to acknowledge the problem. Mine didn't develop bubbles, just cracks.
On ours, I drilled very small holes (I mean tiny), at each end of the cracks to prevent them from getting longer. Then took some Dicor Self Leveling Caulk and spread some over each crack, forcing it into the crack. After the Dicor had set up, I covered the crack area with a strip of 2 inch wide Eternabond.
Did these repairs almost 3 years ago, and everything is still fine and watertight.
If yours is still under warranty I would contact FR and start the frustrating procedure to get it fixed.
Steve & Cheryl + Charlie & Casey, our furry kids.
2012 Forrest River Lexington 283ts
Toad, 2013 Subaru XV Crosstrek
Camping trips this year so far...8