Back when I was working in heavy duty truck shops, we mounted bodies on cab/chassis. I don't ever remember any springs being between the frame and the bed or body. I just looked at the last body builder's guide (1985 Ford) and there is no reference to any type of suspension for mounted bodies. I do remember that the representatives for the bodies that we installed, would specify the type and size of the wood that we used between the frame rails of the cab/chassis and the body.
Most of the time, we just threw a U-bolt around the frame rails / wood spacer and the bottom rail on the body or bed.
As for hauling on large objects, I owned a 4 truck fleet that was hired on to a carrier as part of their heavy haul division. When we hauled Dromedary boxes (basically like having a camper on a flatbed), we had to cross chain side to side and front to rear. I can remember having to take those drop deck trailers being put through some real torsional stress, when delivering the "Droms" to some site that required being off-road. Of course, "Drom" boxes are a little tougher than a camper, but we never used any type of springs or suspensions. As a matter of fact, the worst things to haul were vehicles with suspensions on our drop deck trailers, lo-boy trailers or even flat bed trailers. Even though we also cross chained the vehicles, they would bounce and flex on their own, which caused the chains to loosen and we would have to go back and tighten them.
I believe that if the structure is built secure enough, and mounted to the bed, you should be golden. At this stage of the build, you might want to think about having two rails under the camper and mount it to the chassis similar to the way that commercial bodies are mounted. ... Just a thought.
ON EDIT:
I just happened to find this video on another thread. At about the 13 minute mark, you will see that they have their camper bolted directly to the truck bed. I also like their jacking system.
Aussie Camper
