A few points to add...
1. Not all floorplans and trailers are created equal, some of the are stable to begin with, the others are better be towed ones to the park and left there permanently. The fact that trailer sways just because of speed will not give any prizes to the chassis. Spread axles of the Passport is not a bad thing. In any case I would expect the dealer to help with setup.
2. Hitch weight is relatively unimportant for stability of the vehicle is balanced overall. I have 11% maximum and the trailer is as stable as a fifth wheel if everything hitched right. Can hit 73-75 mph without any sway, but usually go 62-65. In Europe people tow with 7-8% hitch weight and have no problems.
3. Throw away your old hitch, get one with up to 1000 lb bar rating. 550 is totally useless for this trailer.
4. You need 2 sway control bars. Period. They are not band aid. They just prevent sway from starting if set tight enough. I tried to go with one only as well as with no bars and the difference is huge. I would say that proper weight distribution and vehicle balance are essential for safety and drivability, but sway control is essential for comfort so you will not feel any oscillation of the trailer
Gene//////'16 Passport 2670 BH
'11 Mercedes ML 350 gas, Reinforced OEM hitch receiver,1000 lb Eaz-Lift with custom welded head, 2 sway control bars, P2, tst 507 trailer TPMS