I don't know if this will answer your question or not.
I have 2 thomas built busses, a rear engine diesel pusher, and a front engine gasser conventional nose. Both have a medium duty truck style "ladder" frame. Two major frame rails, with lots of cross members. The frame rails are C channels.
Upon these frame rails, the floor of the body appears to be constructed of a lot of bent steel sections that run perpendicular to the frame rails. These sections are also C shaped, with an approx. 1" lip on each end of the C, and the back of the C being the surface of the floor.
I haven't gotten far enough into the walls of either bus to see how the walls and ceiling are constructed. However, from the conversion threads I've read on here, it appears that there are ribs running from the ends of the floor sections up and over the top in a hoop configuration and back down to the floor on the other side. These hoops run up between the windows. I'm not sure of the exact configuration though.
There is an inner and an outer skin for these walls, that helps form a hollow box, usually filled with insulation. The skin is typically steel, and I've seen a variety of attachment methods, from riveting to torx screws to standard phillips head screws.
Down the outside of the sides there is a "rub rail". This performs 2 functions, one being to help protect the skin in the event the driver scrapes something, and the other is to add extra rigidity to the sides. As a side effect, these collect grime and water, ruining your attempts to paint the vehicle.
Upon the roof of my 2 busses, there's also an area about 2' or 3' wide, that has ribs on either side. These like to hold water as well, and I think adds some strength in the case of a rollover event.
I hope this information is helpful in some way,