I believe the sensitivity of the frame and body in torsion is a major issue with DC, and pointed out in their body builders data. Typical truck frames are channels, which do not have much torsional stiffness, so bodies that are mounted are often done so with loose or spring held high deflection mounts....look at a concrete transit mix truck.
The sprinter frame members are closed rectangle sections, so stiff, but not really much contribution in comparison to the body itself. There are 'closed' or tubular portions at the body bottom, or skirt, that probably add more. Cutting an opening, say on the roof, or modifying a member that carries twist can cause lots of other points of flexing stress, often with concentration factors that make ugly problems.
MB offers dual sliding doors, which pretty much eliminates the body envelope itself as stiff, unless doors are shut and wedged, so my guess is that the lower body skirt sections are the most sensitive to avoid compromizing. Note that the cab chassis comes with a second 'frame' on top of the original to give a minimum frame competency, probably with little regard to any stiffness added by built on bodies.
Some american vans, and pickups with open box beds (again no torsion) have had problems with torsional oscillation and vibration (NVH in parlance). You can often see and feel this riding in the back.
Doors help close the back section and will take loading when the body twists and deforms...maybe another place to watch for wear and adjustment.
Dale Ronsin
dronsin@earthlink.net