The non-ability to simply run a wire through foam sandwich has been adequately IMHO. I had a job in the late 80's in which it could not be avoided and the unit was a near new Alpenlite thirty footer.
The load was 30 amperes and the owner insisted on not running wire under the unit. Flexible waterproof conduit never has been cheap and it wasn't even back then. So the owner stripped panels and cabinets off the walls (side opposite the refrigerator). Because of the length and because of the insulation factor I decided to use 6AWG wire. A router was used making parallel grooves and where the wall met bulkheads, a sharp wood chisel was employed. Splotches of canned foam were applied to adhere the cable into the foam. It took him over a week to do the panel removal and replacement. As an aside he discovered really stupid manufacturing defects like multiple staples punched into the 120 vac wire in the bathroom. My decision was and is this is a big - big job. His forté was to run an inverter and HAM radio in the bedroom via a Trace 812 inverter. He ganged eight Trojan T-105's in the front cargo area under the 5th wheel overhang.
I wired Quicksilver with four once inch PVC tubes per side, with wiring breakouts to avoid this problem. Good luck with your project. A near to the batteries inverter is the way to go.
