In the cabin I've got "shag carpet" going up the sides where the walkway is - below the front of the couches and up around the cooler holder. . Is that finished fiberglass underneath the carpet or is it unfinished?

mine is finished

Interesting. That would be a much nicer look than dingy carpet. But I think my carpet goes up under the sofas to. I need to go over and look I guess.
I don't always trust that any two of these things are the same......

yours had had a few owners too, at one point someone probably thought that it would look good.