Ceiling / roof composition

[genre]

Hey folks,
Seeking the more experienced do-it yourselfers help.
I would like to install an extra light in my pop-up.
I have a roof vent with a fan.
I was thinking of running a wire from the fan leads to the new light fixture. Maybe 6 inches away.
I would like these wires to be hidden.
I have removed the flashing from around the vent exposing the wires.
Now if I drill trough the first layer of the roof (where I would place the new fixture), will it be hollow or filled with styrofoam?
If it is filled in I guess it may be impossible to feed a wire.
Any suggestions!
All help / ideas are welcome.
Note my pop-up is a 2008 Viking Legend 2170 st. But I am guessing most roof construction would be similar. (hollow  etc)