Why shouldn't I buy a 2015 Lincoln Navigator L?

The limiting factor on the Navigator/ Expy with the tongue weight has nothing to do with the hitch. It is the rear suspension that limits it. That's why I say 5500 pounds dry, add 1500 for loaded, 7000 pounds means a tongue weight of 910 pounds roughly. Add a heavy hitch like the Hensley and you lose tongue weight and payload.