Probably it's plywood over the metal frame, and the roof membrane over that. Attaching the brackets to the plywood with screws should be perfectly sufficient in my estimation. If you can hit some of the aluminum supports, so much the better.
I wouldn't bet on longitudinal supports except maybe around vent and air conditioner openings. Often I believe it's just a series of trusses/bows spaced every couple feet or so, as I understand things. I'm talking fairly generally here; I have no idea if Jamboree did things differently.
You might be able to see exactly what the structure looks like by removing the inner parts of a roof vent and peering around.
