Truck bed camper shim

I just use air bags to level the camper/truck.

Photo before and after might help.

Is the truck level and the camper not???

My HD sits low in front when unloaded, and level when camper loaded.

Truck is level, camper is not. I'll have to look up posting pics, but the 42" piece has made an improvement. I just wonder if it's too long and the camper may teeter on it?

1131 is a long camper like my 1191 and hangs off the back quite a bit. Is it possible that you have rot in it making the back sag that hangs off?

Rot on the camper or truck? Truck has none, camper is unlikely to have rot being from CO but anything's possible.

Ours always looked tilted too & it made no sense. Finally figured out something. Take a 5 ft board & put it across both bedrails, just behind the back window. Now take a measurement from the bottom of that board to the truck bed. Now take that board and move it back just forward of the tailgate & take that measurement again. On our 2011 F350 there was almost an inch difference if I remember rightly. Basically a manufactured optical illusion. No idea why, so I just got used to it.

It is not a good practise to support only part of the TC bottom on the truck box floor.
When you do part of the floor is unsupported and could sag and could develop weakness and cracks.
I use a slab of closed cell construction foam. 100% coverage. 4x8 in my case. Easy to cut if need be.
Cheap and waterproof.

Have the old lady move to one side or the other to level it out.

GM truck boxes are not flat, they have a peak across the bed by the wheel wells to allow water to drain from the front and rear of the box. Campers will rock on this peak unless a shim is placed in the rear. I used two semi truck mud flaps under my rubber mat to make a level surface. Use a straight edge, like a 4' level and slide it on the floor of the box from front/rear and you will see the peak.

My 2006 Dodge RAM diesel short bed is the same way; shallow at the cab, deeper at the tail gate. It really bothered me, the way it looked so I did something about it.
I used a full length rubber bed mat made for my short bed truck and cut the front 1/3 off and laid that over the end close to the tail gate. This made the bed a 3-step level with 2 layers of rubber mat at the tail gate, 1 layer in the middle and zero layers near the cab. This made it so the camper was now even with the side rail of the bed. Measuring between the camper and the top of the bed is now within 1/4" front to back. The rubber flexes and gives so the load is pretty much even across the bottom of the camper.