Some of what is confusing you, is in the fine print. Nissan says your vehicle has max tow capacity of 6000 lbs. It can, as long as there are no passengers, aftermarket accessories, or cargo. That's not how people go camping.
When you add weight with people and cargo (a weight distributing hitch counts 80 - 100 lbs against cargo weight), that 6000 lb max tow weight, along with your available payload are going down, pound for pound. The trailer tongue weight, also counts as cargo weight.
Look at your tire and loading sticker (on drivers door post). It will have a number for max occupant / cargo weight. That is the amount of weight the vehicle was rated to carry (as it left the factory). Any weight (accessories, people, cargo) added to the vehicle from that point, reduces the available payload and max tow capacity.
When you hang a WD hitch and tongue weight on the rear of the vehicle, it adds weight to the rear axle and takes weight off the front axle (like a see saw). The primary purpose of the WD hitch, is to use leveraging to restore some, or all of that lost weight on your front axle.
Too much weight lost on the front axle, can cause unstable steering and trailer sway. Your owners manual should tell you a percentage of front axle weight restoration that is recommended.
Your user guide is saying, you don't need WDH if the trailer's loaded weight is under 5000 lbs. You're on the edge, your call. The class IV hitch, mentioned in your user guide, is not the WDH itself. It is a heavier duty frame mounted hitch, on the tow vehicle. The WDH mounts into the class IV hitch receiver.
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