I did the same cabinet repairs to my 87 Lil Bigfoot
. The original wood was glued to the fiberglass. I don't know what the original glue was but it eventually failed. I won't be storing anything very heavy up there anymore.
I researched other modern adhesives and settled on PL Premium (Loctite or Lepage). I pre-tested some on scrap wood and fiberglass. It seems very strong and it's lasted 3 years so far and I travel lots of bumpy roads. Time will tell, but I am pleased with the result.
If you want a more flexible adhesive, I have also used E6000 Industrial Adhesive with very good results. They claim it is 2X stronger and 60X more flex than Polyurethane.
Whatever you use, strip off all the old adhesive from fiberglass and wood strips (if you re-use original wood strips).