Don't pay a lot of attention to advertised towing capacity. You'll run out of payload before you get close to that number. When manufacturers calculate towing capacity, they don't allow any weight for aftermarket accessories, passengers, or cargo. As you load accessories, people and cargo, your available payload and max towing capacity are going down, pound for pound.
Your true towing capacity is limited to the weakest link in the combined ratings on the truck. That weak link is normally payload. A weight distributing hitch and trailer tongue weight are counted as cargo weight on the truck.
When you look at trucks, open the drivers door and check the tire / loading sticker. It will show a number for max occupant / cargo weight (payload). That is the truck's capacity to carry everything and everybody you put in it or on it. That number will vary from one truck to another, depending on installed options.
Example:
Say the sticker says max occupant / cargo weight is 1200 lbs. If you have 500 lbs of people, 200 lbs of tools and stuff in the truck bed, and 100 lbs of weight distributing hitch, you only have room for 400 lbs of tongue weight. That would limit loaded trailer weight to about 3000 lbs. The same truck with only a 150 lb driver and the WD hitch would have room for 950 lbs of tongue weight.
Check the rating on the hitch receiver. It will have max weights with and without a weight distributing hitch.
The closer you get to max weight on payload or towing capacity, the more unpleasant the towing can be.