Fiberglass vs Metal

Honestly, at that age, current condition is the key- if there is no delamination now, just keep it sealed up and you'll be fine. The Mobile Travler of that vintage is a fine rig, the 460/C6 combo is good for 200k miles, only downside is above 60 the back two barrels open up, and you get about 5 mpg. I got tired of rebuilding the motor craft/Holley carb, and installed an Edelbrock performer, simple bolt on switch.

Fwiw, I had a customer who was in an Airstream that got struck by lightning, gave him a heart attack (no kidding), blew a hole in the side of the trailer.