Chevy or GMC Truck would only have the wiring if the truck had the "camper wiring" option from the factory. It's not a standard item, but the trailer plug at the rear bumper is standard on 99+ trucks.
If the truck has the camper wiring provision, the wire harness will be tie wrapped to other wires between the bed and cab, basically on top of the frame rail. You can see a bundle of wires when you look between the cab and bed.
If the truck doesn't have it, there's aftermarket options available that are plug and play, which don't require permanent modification of the factory wiring.
The factory camper wiring harness is an add-on harness which Tees in to the trailer wire harness behind the spare tire area. When I sold my 2002 2500HD, I pulled the factory add-on harness out and put it on my 3500HD, which didn't have the harness.
The best recommendation, for future options, is to use the standard 7 pin trailer receptacle in the bed and put a plug on the end of the Lance harness - that way the truck is ready for anything - 5th wheel, goose neck, other campers, etc.
Once I find the right part you need at E-trailer, I will post it.
Bed receptacle: (not saying it needs to be mounted at the rear of the bed, but I think that's where most 5th wheel owners mount it so they can easily reach it.