The weights given do include fuel...a full tank.
Payload is GVWR minus base weight.
Base weight includes full fluids. This includes a FULL tank of gas.
Payload is anything put in the truck. People drinks food accessories.
Another misconception is that 150lbs is allowed for each passenger. This is also wrong. There is no allowance for people with the payload number.
The 150lb passenger allowance comes into the max towing figure.
The max towing figure allows for 300lbs cargo and passengers in the truck.
The reason max towing is usually unattainable...most families weigh over 300 lbs and carry cargo with them
There is definitely capacity left on the table...and it is fine to try to use it...but the truck will not perform as intended.
I'd also like to add that the factory "rake" on the truck is by design.
It's so when load is added, and the rear of the truck starts to sit, the brakes, steering geometry, are all working on the right plane.
As we load it further and further, it squats further and further. It's essentially telling you to "stop"!
This is why leveling kits are personally not for me. If It's level with nothing in it, that changes pretty quickly once something consequential is actually placed in the back. This is unless you have an airbag setup, or the shocks have a "payload" setting or something to that effect. There are certainly exceptions, but for the most part many of the "levelled" trucks I've seen are now starting to teeter skyward with as little as 300lbs in the back. Add more, then add in Mass X Velocity^2 when going over bumps? Bad juju. It isn't designed to handle the towing and hauling with the nose in the air and weight lifting off the steering wheels. If It's bad on level ground, certain scenarios (like bumps, road imperfections, speed), can make it downright dangerous.
Just my opinion on that. As I got the truck to occasionally have 800-900lb loads in the back, leveling kits are not for me. If It's level now, it will only sink 900lbs later.