You guys are too kind. Thanks.
To make the side walls I installed 1"x2" furring strips down the side of the bus with sheet metal screws right into the sheet metal inside walls of the bus.
Then I installed some fiberglass insulation and put 1/4" thick luon paneling over top. I used luon because it is cheap $10 per 4'x8' sheet and it is smooth.
I covered the walls with a thick, textured, vinyl wallpaper using professional adhesive. The thick, textured wallpaper covers a multitude of sins. The wallpaper isn't paintable. It has a color, but it almost matches the existing paint on the bus roof.
The partition walls are 3/4" MDF (medium density fiberboard.) MDF is really rigid and can stand up on it's own without any studs. That way I could have sturdy walls that were only 3/4" thick to save some space. I covered them with the same wallpaper.