pivoting frames and mounting campers

Please excuse my simple answers as I'm not an engineer... My knowledge only comes from being around expo trucks for many years and working together with one of the best Expedition Truck Engineers in Germany.
Anyhow:
Every truck manufacturer has a "truck body builder" guideline. Within that guideline (together with tire sizes and required clearences) the manufacturer is telling you where you can mount truck bodies or other accessories. Most of them only talk about commercial truck bodies. Mercedes and MAN for excample have special guidelines for offroad trucks. To my knowledge, only qualified outfitters are able to get that info.
In other words, trying to figure out by yourself how to build a subframe that will protect your truck and the camper body is not easy and could be potentionaly cost a lot of money in repairs.
My advice is always to hire someone that has designed and build them in the past AND can proof that his designs are without fail.
 
Twist...

I've been trying to work out the best way to mount my body, and have been looking into 3 or 4 point mounting and how and where to mount the points... I think the easiest way to describe twist is to find yourself a 300mm/12" long standard plastic ruler... Twist it over its length (like a truck chassis off road) and watch what happens... The center line is the only 'plain' that stays in the same place. The left edge and the right edge stay relatively straight but not in the same plain. They work in opposite directions. With this in mind, wherever you mount your body on the outside edges of the chassis will be the section that the body will follow. If you want the cab and body to have minimal twist difference then your two outer mounts should be close to the cab... (but you must then allow for lots of chassis twist clearance under the rear of the body, especially in the rear wheel arches where the tyres will be). Going back to the ruler principal, if your cab is mounted at the front (1cm point), and you have two outer chassis to body mounts so the body stays upright (around the 12cm point on the left and right edge on the ruler), then your 3rd and or 4th body mount can only be and must be in the center of the ruler (around the 20cm and or 30cm point). This is not possible on the ruler, but it is possible on a chassis. One mount could be just in front of the axle, and the second could be at the rear... giving you a 4 point mount that spreads the body weight over the length of the chassis, whilst letting it flex like it's designed to. This is exactly how I plan to mount my body on my Merc 1222. I plan to have my two front outer mounts approx 24" behind the cab, with the 3rd just in front of the rear axle, and the 4th about 36" behind the rear axle. The only thing I plan to add is two sprung cab shock absorbers between the outside edges of the chassis rear and the underside of the body. These will give the body a bit more support when on highways (hopefully lessening body roll), but being soft enough to compress and stretch allowing chassis twist when off road... Attached is a pic that I have borrowed from a book I have :-

https://haynes.com/en-au/build-your-own-overland-camper

If you have not seen this book, buy it. It has so many things I've not thought about, including the pic below and a section on chassis twist and body mounting. It clearly shows the chassis center line and also the deflection if you don't get your mount smack in the center of the chassis. Hope my different approach at describing the twist makes sense, and if needed I'll sketch my mounting plan and post it here for comment/criticism/praise! ;)

Chassis twist.jpg
 
And that's why I only use bushings, as opposed to bearings.

Even when designed correctly, there is always a chance that certain deflection can cause binding.