If thinking about metal-framed construction, visit Newmar (Nappanee, Indiana) for a factory tour. They tour twice a day, and when I did it, the tour was comprehensive.
Newmar is one of the few RV manufacturers using metal framed construction (as differentiated from laminated panel walls, which get called "aluminum frame" even though they are not framed at all). While currently building only motorhomes, Newmar used to build their towable RVs the same way, from the frame up.
Northwood (Nash, Arctic Fox) builds high quality wood framed RVs, skins them with aluminum or Filon (standard on the Fox). But their plant is more distant from PA than is Nappanee.
Most factory tours I've taken have been for manufacturers doing panel wall construction. This is typically a perimeter frame of aluminum tubing set into a foam panel the full size of the wall, with some widely spaced verticals and reinforcement plates, and framing around critical openings. The skin on both sides gets glued full contact, and the strength of the wall derives from the stiffness imparted by the skins. These pieces then typically get screwed to the floor, roof and ends through the aluminum tubing, though some manufacturers do a little better, e.g. interlocking extrustions rather than just square tubing.
This is something you can maybe do at home, but at the factories there is considerable capital investment in tools like framing jigs, jigs or robotic cutters for routing channels in the foam and cutting openings in finished walls, and bonding equipment (usually either vacuum tables or pinch-roller presses).
Building up a framed wall, as if building a lightweight house, your costs will be labor, not capital equipment requiring some volume of production to pay off.
Another way to build would be a moulded shell, like a boat hull. In the early days of fiberglass boats, there were many home-built hulls, plans and kits sold for the project. But first you build the moulds, then the hulls. Usually the moulds cost more than the hull, so it would be nice doing more than one.
