Maybe all isn't lost
Not sure how bubbled and wrinkled the EPDM is, but you may still be ok. I did my roof a few years ago and had a few wrinkles and bubbles. The material has eventually flattened out. The material is like a plastic bag. It isn't critical that the material is bonded to the underlying substrate. It is critical that that it is sealed all the way around.
We used EPDM to line canals in Arizona. Major area was laid over decaying concrete 25 years ago. Still in good condition and keeps water from seeping out. The EPDM is a very tough material. It does deteriorate with UV exposure, but will outlast your fiberglass wall panels.
Al & Sue w/T-Bone & Rosco
Go Devils Bus, 1994 8.3 Cummins, Spartan Chassis