I would advise caution with the whole idea.
Those roofs are often 3/4 x 1" lumber, so drilling 3/8" hole is like taking 30% of the load capacity, what is not too big to start with.
I am carrying canoe on my roof, but I made long support that attach to factory railing.
Other thing is practicality of roof mounted panel.
I camp mostly in Sierra on lots that are 90% in the shade, so roof mounted panel would not work for me at all.
I am carrying single panel and 50' of extension wire for it, so can move it to spot with direct sun and by repositioning it couple times get max charge.