Aye. I never really studied it until now...
I removed the trim piece from the 14" roof vent and poked a light in there. It's got what looks like 1x1 wood stringers, fiberglass insulation between the stringers and a thick layer of fiberglass inside and out.
I can see a couple of front to rear stringers, can't see if there are any cross ribs and didn't have any long stiff wire handy to poke around with.
The sides are about an inch thick and then it gets thinner about 2" up from the mounting flange. Looks like an inner and outer shell which are glued together in a 2" band around the base with the mounting flange sandwiched in between.
If I roll this thing and end up topside down in the river maybe I can just poke a paddle out the window and...
nevermind.
I also discovered wiring that runs up from the fridge cabinet and is capped off and tucked back into the top behind the vent trim. I'm guessing it originally had a vent/fan (not a light, there is a light a foot away) and at some point it got replaced with just a vent. Good news as I was thinking of replacing the vent with a vent/fan and now don't have to fish in the wire.