No definitive data on thickness of the roof structure that I can find -- but look at the vents/exhaust fans and the skylight in the shower to get an idea. My guess is the framing is 2 inch square or less...
What are you trying to mount? How heavy/bulky is it? Is it going somewhere that a previous device was mounted so you know that there is some structure there? (I replaced a crank up TV antenna with an electronic/automated one so that mounting spot was a known braced area...)
Sealing the fastener penetrations against water egress is the most important thing after strength considerations.
2007 Tour Master T40C
Acura MDX toad
"It takes a great deal of time to recover from any improvement..."