I have a 2012 Rockwood 2503S.
The skylight was just replaced under warranty, but before taking it in I saw that other folks had skylights replaced and had posted some photos. Check the skylight threads and you'll find those, but I took a look at mine before taking it in for repair.
At the skylight mine is a thin sheet of rubber then something like 1/8" of some type of fibre board, thick styrofoam, 1/8" of more fibre board, and vinyl ceiling sheet.
No plywood. Screws are held by styrofoam, at least at the skylight. There might be blocking (at least I hope) where heavier roof gear is currently mounted - I couldn't say.
Perhaps some other folks have have seen something different.
Fred, Gail and Sandee the Puggle
2008 Pathfinder, Native Ultimate 14.5, Delta 10
2012 Rockwood 2503S