Here is a picture of the slug I cut out of my rear wall when I installed my shore power inlet. The wall thickness is 1 1/4" the other 2 pictures show cutting off the tip of a Zip-It wall anchor and the installed anchor. I used the pilot hole from the hole saw which was oversized from the nominal 1/4" pilot bit size so if you were drilling new, I'd probably use a 17/64 or 9/32. You'll need to carefully control the depth of the pilot hole so you don't poke through the outer skin and snip the tip of the Zip-It right where the threads start.
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.