I’m pretty sure the issue being asked about has nothing to do with the suspension, or the truck sagging.below level.
It’s the optical illusion that the camper is actually sagging at the rear IN THE LEVEL TRUCK BED.
It’s the illusion the the camper is sagging relative to the truck, not the whole truck sagging relative to the ground.
And it’s an illusion. You need to measure it yourself… get a straightedge, board or whatever that reaches left and right across the tops of your bed rails. Measure from it down to the bed floor at the front and at the rear.
I’m pretty sure you’ll figure out why the rig looks the way it does.
You do not want to put a board under it at the rear, because the camper is built to be supported sitting flat on the floor. You’ll put all the weight and stress right where the board is, which isn’t good.