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.