I'm pretty sure it's a fiberglass top with plywood underlayment and a layer of foam under that....maybe another plywood layer. At least on ours, there was an interior roof trim covering it all that looks like it was made out of textured aluminum. I tried to install some of those puck led lights over the shower once....screw stripped. There were, however, several things factory mounted that were screwed in, so it can be done...just be really careful and don't expect it to hold a lot of weight.
2012 Chevy Tahoe LTZ - HD tow package
2017 Rockwood 2703WS Emerald Edition