Here’s a power feed box I designed to get 120V AC from an extension cord into the van. It used an outdoor box and waterproof fittings to get to an existing hole in the van that leads inside.
Completed, with wire and cover plate. The wire is flexible cord 12/3
This is a closeup of the power plug under the cover. It is connected to an outdoor outlet on the house by a regular extension cord
The outside unit is attached to a sheet metal flange at the back of the truck.
Looking from the back of the truck, it’s hardly noticeable
Inside, the wires coming from the plug box connect to an AC outlet.
Later, a system to feed power to a few outlets in the van will be added!