As Puglsyyy has rightly noted, there is a significant difference between the box on a goods truck and that of a camper. The same goes for a service body, as Kerry mentioned, where there is normally a combination of smaller boxes that are attached to the tray independently. Smaller boxes take up less area and will generally be structurally stronger, so flexing will have less of an affect on them.
I have looked at a lot of our Rural Fire Service (RFS) trucks and they normally utilise a spring mount system between the chassis and tray and often also rubber mount some of the equipment on top of the tray. The designers of these vehicles are obviously aware that flexing can destroy equipment, so do what ever is needed to ensure that damage is reduced to a minimum.
Keeping a camper box from twisting is a whole other challenge, simply because of its physical size. If the camper is a pop-top it is even more difficult, as that type of design is less structurally sound.
At the end of the day, the mounting system used for a camper box on a truck, like the Fuso or Isuzu, needs to take into account the terrain and use that is likely to be experienced.
Hard mounting a box to the chassis of a truck is a cheap, tried and tested method and has been used for generations. However, comparing a goods truck to the requirements of an expedition camper is really an apples and oranges scenario.
Stress on metal is a fickle beast and will not necessarily present itself immediately. Stress damage is normally not easily visible either, so a simple visual inspection is unlikely to identify stress related issues, until there is an obvious failure.
The reliability of a vehicle becomes much more relevant if you plan on taking it into remote locations, where support may be limited or non existent.
Murphy's Law dictates that that failures will normally occur at the most inopportune time when you are least able to deal with it.
I see a spring mounted camper body a bit like an insurance policy... you may never need it, but if you do, it is there to do its job.
Only difference in design would be use the vibration setup. Everything would still sit on the frame rails.