Trying to understand payload

You are absolutely correct. Your payload is the trucks GVWR with the ACTUAL weight of your truck subtracted from it. It includes EVERYTHING you put in the truck. Talking about ACTUAL payload not the brochure payload or even the door post payload. Put a gallon of milk in the truck and your AVAILABLE payload is reduced by approximately 8 lb. ACTUAL payload is the number you need to concern yourself with. Load up the truck with whatever you travel with, weigh it and subtract that weight from the GVWR and you get your ACTUAL payload.