With my trailer freshly insulated (polyiso), and BEFORE AC wiring, I fit my new luan ply walls and cut the outlet holes where I wanted them. Then I used a magic marker to mark the box holes, most of which were going to be up against studs so the box would be screwed to them.
With Luan removed, I ran all my AC wires along the top edge using nylon ties and screwed in tie mounts (dont rely on Stick Ons), creating organized bundles. Where wires had to drop down, I would cut out the strip of aluminum from the polyiso, then use an old soldering iron to melt a groove in the foam down to the box cutout. Then use aluminum tape over the wire. No other support necessary, and the wires are never near any metal to rub thru.

- Construction Wire 20180429_074056b.jpg (207.08 KiB) Viewed 882 times
Dropping down from the top is the best way, leaving yourself an accessible area for future additions or repairs. That top section can be upholstered with a trim panel easy enough, or, like I did, the edges of the ceiling are a smidge lower on the sides.