Not just a box...
The vehicle is typically referred to as a "box truck." The "cutaway van" chassis is ordered from your choice of vehicle manufacturers, typically Ford or GM/Chevy. We chose GM for no particular mechanical or automotive reason. Specifying the vehicle parameters was very straightforward. The GM dealer then becomes the "general contractor" for the package to be supplied by the body/box manufacturer.
Then you choose the box that mounts on the vehicle chassis rails. The chassis was manufactured in Detroit and shipped to Supreme in Goshen, northern, Indiana. This appears to be the center of the RV and truck upfitter universe. Most of the coordination that went into the ordering process was done directly with the body manufacturer, and that is a long, separate story. The basic box was fabricated and mounted at the main factory, and the initial installation of the generator was done there. The skirting, storage/toolboxes and rear bumper fabrication was done at the body company's local, St. Louis facility.
The various holes, or penetrations, for appliances, vents, windows, etc. were part of our final construction.