The "hybrid" system GM put into your particular truck is a "mild hybrid" system, which has an oversized alternator built into the transmission flywheel housing. The Alternator's large size allows it to be used also as an engine starter motor to crank the engine.
That extra-large alternator can be turned by the running engine to generate a lot of electricity-- Enough to drive the four 120VAC outlets in the back.
If you want to use the 120VAC outlets in your Silverado, you leave the car running and plug in your electrical appliance. The engine will spin up the alternator to produce the 120VAC power whenever it's needed.
Hope this helps.