The critical factor is chassis deflection, put one rear wheel up on a jack and see how much twist the chassis takes. Ideally the chassis should be loaded up with a weight that comes close to the body when fully loaded.
You can get a rough idea of how much flex you'd need between body and cab by putting a wooden frame of the chassis and watching it move as you jack up the back wheel.
You can repeat for all wheels and see what results you get.
Generally the chassis under the cab is more rigid, the engine and gearbox mounts see to that. So the compromise is to have the body move in relation to the larger chassis movement at the rear whilst staying close to the cab flexible connection at the front. Is your cab suspended?
Using differently sized air bags to support the body works ok, I've seen that on sleeper pods on road trains. Use small movement bags close to the cab, big movement bags at the rear, but able to lock them down for highway use. Tuning and selecting would be the hardest part, how much pressure, how much movement, etc etc. You also need shock absorbers to stop the body moving to rapidly.
The other issue is how much flex can the body take - 5cm of movement onto a welded steel joint will cause failure eventually, a lot of refrigerated truck bodies still use rivets to secure corner posts - but truck bodies have an economic lifespan far less than most peoples expedition vehicles lifespan.
Sealing between cab and body is a real pain for tilt cabs, if you need to tilt the cab for routine checks you'll soon get pissed off with having to undo 40 press studs around the seal - or whatever seal you decide to use. If its not tight it will leak, tight and flexible are hard. Water may not get in but dust by the bucket load will.
BTW wood is a forgotten treasure for body builders, wood flexes, its easy to work and long before low weight composite bodies came along it was the only way a van body got built.
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