If the underneath is still sound, the skin can be re-glued. For small areas you can drill a hole or two and inject an adhesive, but for larger areas you may have to cut a slit in the bubble to get it to lay down again, and to provide access to spread the adhesive around. And if you need to make repairs to the backing, you can sometimes cut away a rectangle of the skin, do the repairs, and then glue the skin back on. Of course, the holes or slits have to be filled with an epoxy filler or a gel coat repair kit.
The adhesive can be a urethane construction adhesive (it needs to be a thin, spreadable type) or an epoxy resin of some kind. Contact cements work well if you can coat both surfaces and align the pieces correctly and press together, but don't provide any working time if you need to slide the pieces around a bit. The contact bond is instant!
There are products for filling and sealing rot-damaged wood underneath, e.g. Git Rot. Marine stores have more products for fiberglass repair than RV dealers.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL