I would think that hard rubber stops/mounts similar to what you would find on a leaf spring suspension to prevent bottoming out would be your best bet. You do not want the empty air bag to support the box weight. (air bags do not like to be over extended either) You will defiantly want some way to limit any side to side movement. Not thinking a plate with slots is going to work very efficiently. There will be a lot of motion going down the road, and you will need some type of bearing surface to prevent wear on the plates, and or lots of noise coming from steel on steel. some type of links with bushings or heim joints may work. Take a look at how some of the trucks are mounting their sleepers, especially the unitary cab/sleepers. Most of them are supported by air bags. Some of the older rigs had a hinge point for the cab way up front and the rear of the cab and sleeper was on bags, this gave a better ride but still had some side to side stability due to the hinges. Not very practical for a long box though. Also consider adding some shocks to the set up, an air bag is basically an adjustable spring, and will bounce like crazy without a shock to dampen it. The amount of air in the bag can be controlled with a ride height valve similar to what is on the rear suspension of an air ride truck, a lever is moved up and down as load is increased or decreased and the valve its connected to will regulate pressure in the bag to keep ride height the same. This can also be done manually with a regulator which can be adjusted up or down as needed depending on weight. This can work well if weight is not constantly changing. , and with an RV box might be simpler than an automatic system, unless you can pirate one from a junk yard truck suspension.
I have been contemplating some type of air bag setup for the conversion I am planning. As of now, I am still leaning toward the tried and true hard mount to frame rails. I am concerned with coordinating movement between cab and box, especially if both are on air. Not sure I would want to bolt/weld/Velcro the cab and box together into one long unit either, so much twist and torque going through the frame when going up driveways etc.
Keep us posted with what you end up figuring out. Always looking for a better mousetrap.
Good Luck
Dave
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