At this point, you are looking at dangerous ground. Generally they give you the maximum towing capacity with one operator in the vehicle, so that the weight of your truck loaded, and the maximum weight of the trailer will not exceed the Gross Combined Vehicle Weight.
Now the catch, how much weight are you actually carrying in the truck, and yes that includes the passengers, the weight of the hitch, the stuff in the pick up box too.
Then are you implying that you will only load a trailer to 400 pounds of goods?
Then the trailer weight is often the bare bones trailer, and doesn't include some things that are optional extras for the build like the A/C, extra battery, TV and others.
So you see, adding 1000 in the truck takes away from what's out back. It will pull fine, but will it stop? Will it respond correctly in an emergency?