If you're wondering about the bolt length, are you assuming you will be pinching the two plates between the outer and interior walls, therefore needing longer bolts? If so, that install won't work because the foam insulation between the outer skin and interior wall will just crush as you try and tighten down.
When I installed a Firestik NGP on my Georgetown, I cut an opening, no larger than what a wire mold electrical box would cover, to expose the inside of the outer plywood wall skin. Drilled my holes and mounted the antenna bracket. The bolts that came with were plenty long enough because you're now pinching between the outer plywood skin only. After routing the cable to its desired path, I covered my hole with the wire mold electrical box and cover and it looked beautiful and professional.
Take care in what you do with any leftover cable, following Firestik's recommendations, so you don't screwup your reception.
Hope this helps...!! Post back and let us know how you made out!
2005 Georgetown 342DS XL