Making progress!
You should be a little concerned about welding close to the skin. When I welded in my studs for windows I even removed the screws holding the skin and stuck some thin plywood between the studs and trailers skin. At the very least to much heat will bubble the paint on the outside.
For the 110v, I used metal clad wire. Solid wire should be fine I would just be a little concerned about the wire not being protected by anything.. Its probably not an issue, just make sure the wire is strapped well so vibration doesn't wear into the wires insulation..