I think the basic problem is that the truck manufacturers are simply making the truck box sides too dam high. When you compare them to older trucks, it is ridiculous just how much difference there is.
A good welding shop could easily make up a subframe to slip in between the camper frame and the axles. Do not just add blocks to the camper frame, as the side forces will eventually tip them over and tear out of the frame. There needs to be cross-members installed to stabilize that side-to-side movement.
Heck, some factory frames have failed due to no cross-support.