I've seen a lot of photos of the inside of empty vans showing the curved interior walls. Is there a problem with framing the walls vertically? Would more insulation have to be added to prevent a void?
Just off the top of my head, you'd lose a lot of interior space. While vertical wall framing would make it easier to panel/finish, it has to be adhered to the side wall ribs (unless you want to try and pressure fit it top and bottom or secure it top and bottom). Then there's the issue of a rigid wall frame going over bumps and twisting as the frame flexes.
Since so few vandwellers ever try to make the straight walls, that might be a clue...
Maybe a compromise between round and flat is stepped. Maybe utilize the deepest parts for cabinets.
Or just make a flat wall and then use shelving to go out to the curve of the body