I can't speak for Mr.FixitChapman's technique, and I can't tell presactly how he did his cabinets.
There is a simple (sort of) way to get the contour of the roof pretty accurately: use a divider (compass). Cut a piece of cardboard or thin fiberboard that very roughly matches the contour you want to duplicate, and clamp that piece in place nice and square with the wall of the bus. Put a piece of paper on it if you want to make a paper pattern, or just trace on the piece itself if you want a "hard" copy. Using the divider set at, say a 2" spacing, hold the divider in absolutely
the same orientation throughout the operation, and put one side of the divider against the contour and one side of the divider (the marking side) on the paper/template material. Now just follow the shape of the contour. You should have a good template or pattern when you are done. It may take you a few tries to get the hang of it, so working with cardboard at first is best, IMHO. Once you have a fit that you like, transfer the shape from cardboard to something more permanent, and you will have a template that you can use from time to time on different projects in the bus that require a contour of the roofline (in the case of the cabinets pictured).
The way I mount things to the roof is to use short pieces - 4" to 6" long - of 2x2 (1.5x1.5) as blocking. I screw them to the roof along the line of the contour, and then screw the plywood into the blocking from the side. The short pieces will generally follow a gentle contour pretty closely, and they act as the framing of the cabinet. Since they are inside the cabinet, they are not visible when the cabinet is assembled. This picture shows examples of what I am talking about, both on the side wall, and on the contour of the roof.
Here you can see the same principal applied to the galley construction: