Bob,
I first determined the location I wanted to mount the dual DC receptacle. That dictated where I cut a rectangular hole in the back of my cabinet. I put the receptacle fairly high (but not to the top) so the plugs in it would not impede my stored items in the cabinet.
Once that hole was cut, it was a pretty simple process to dig through the fiberglass insulation to find the power wire that feeds the reading light in that corner of the rear cap.
In terms of where to drill the antenna hold, I drilled it inline with the outside vertical leg of the ladder. I did this for aesthetic reasons. The allowed me to tie-wrap the coax down the ladder vertical to just below the level of the hole to be drilled. This ensures water runs towards the ladder, not the hole in the cap. Height-wise, I drilled the hole for the coax at a height that was a balance of near the height of the rectangular hole inside the cabinet for the dual DC receptacle and still being at a point on the cap that seemed to make sense to drill a hole in.
On the previous coach, Terry H and I did all the work. On this coach, I supplied the dual DC receptacle to the plant and they installed that only. I did the rest (antenna, hole, router, amplifier).
Good luck Bob.
Jim