First of all, hello and
Now, to partly answer your question. Recessed heater is going to require "what?" in the way of mounting. Most walls in campers are only 2 inches thick, not the 3 1/2 inches you may be used to house construction.Trailers are built with 2x2 lumber or aluminum in the walls.
As for repairing where you cut through the underbelly, I use thin plywood or paneling above the cut, then pre drill a piece of sheet metal about the same size for the outside. Hold the sheet metal up, use wood screws to mount to the paneling, making the membrane the meat in the sandwich. The I spray coat with automotive undercoating.
As for fishing wires, just like a house, you have to study and figure your best option, and hope it works. You can go up and down somewhat, but never across due to studs. Good luck, and keep us posted.
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT