I sat down and did the math on that sticker in my last Chevy 1500.
Turns out if you add 150lbs for each of the 5 passenger seats in that truck (extended cab, not counting driver) to the recommended slide-in camper weight, it came out almost exactly to the GVWR-minus-curb weight payload capacity of the truck.
The long and short of it is you can use the truck's payload capacity for whatever the heck you darned well please. You can even ignore it if you want.
Whatever's written by the manufacturer is there to absolve them of liability if something bad were to happen. You put 2500lbs of camper in the back of your 1500lb rated payload truck, and something happened? GM can tell you to get bent if it comes to that. That's really all it is.
It's on you to decide what's important, what's safe, what's acceptable risk.
