tsanjose, I can't speak for your manufacturer's construction methods, but what I've seen when working with mine on a Jayco and now the Fleetwood is that they are NOT very thick. I didn't think to measure at the time, but it's between 1.0" and 2.5" thick at the most for most aluminum skin roofs. More, if it's the Coleman ABS roofs.
Check out this
How It's Made episode for a video on how a PUP is constructed. Not ever manufacturer is the same, but when you see how the roof goes together, at time marker 3:16, you see that 1"x material being used with just aluminum top on one side and the vinyl covered 1/4" luan on the inside and you have a lot less comfort when crawling around up there. And that's just styrofoam for the rest of the filler/insulation.