Ladderc06, you can get the specifics of
how a weight distributing hitch works here. Basically, weight applied behind the TV rear axle raises causes the front axle to decrease its weight. The truck's frame/suspension acts as a lever with the rear axle as a fulcrum. The goal of WD is to return the amount of weight removes by the TW back to the front axle. This will also apply some amount back to the TT axles, about 20% for typical applications.
600 pounds dry will be more like 800 pounds loaded, but only 650 left on the truck's axles. While I agree that I have never seen a a pickup pull into a campground with an empty bed, the F-150 and many others are towing very well at or a little above its GVWR.
