The previous owner glued [b]dairy board to the top
It took a LOT of Googling for this City Boy
to understand what you are talking about...
Link: Is This "Dairy Board"???
My 1971 Compact Jr.
had plywood wall paneling glued to the ceiling with construction adhesive when I bought it, it looked like a factory installation and not a previous owner's modification.
In addition to providing a finished surface for the ceiling, the over-sized piece of paneling structurally stiffened the fiberglass roof, [b]as it was bowed up while it was glued in place
. This originally made the roof surface "crown" theoretically to shed rain. I didn't like the look of mine, so I removed it, causing my thin fiberglass roof to sag in the center
collecting a puddle of water in the depression.
I can speculate that your "Dairy Board" may have been meant to perform the same function... Or it may have replaced the original paneling to look more like a conventional "ceiling", and the installer didn't force an upward bow into the board to make a "crown" in the roof.
I just used a putty knife/scraper to pry the panel away from the glue.