When my camper is perfectly aligned with the truck, I have 1/4" clearance on the driver's side between the bump out for the shower and the rear opening. The clearance from the side of the camper to the wheel well matches the thickness of a 2x4. I load with the camper slightly off center. That gives me an extra inch or so of slack when loading. That is still a tight fit but better than trying to deal with a 1/4" of clearance. I then put a section of 2x4 on the left side and lower the camper while pushing in that direction. The 2x4 gives me perfect alignment. I could put another section of 2x4 on the passenger side but never had a reason.
I do not seem to ever see any shifting. Before you look for complicated solutions you might look for the obvious. I have a thick rubber horse mat on the bed of the truck on top of a rough sprayed on coating. No motion is ever going to occur between the sprayed on finish and the horse mat. Under the camper, I have double layers of 1x6 wood slats to raise the camper to the needed height so there is about 2" of clearance between the camper overhang and the running lights on the top of the truck. I have non slip stair tape under the slats. That is like a very coarse sandpaper and there is no slippage between the bottom of the camper and the horse mat.
