I have a 2014 289RK, and I use a Equalizer WD Hitch. I have no sway at all with this hitch.
The only thing I can say about the tank location is obvious and you probably already know, but the kitchen tank is in the rear, and the black and bathroom gray are forward of the axles.
I don't know anything about the blue ox hitch, but I might suggest checking the set up of the sway control and ensure you have the correct weight sway bar rating. (If you previous trailer was lighter, this might be the problem.)
It would sure suck to have to add weight to the front tanks to reduce sway.
BTW, welcome to the forum! My wife and I love our 289RK! its the perfect layout for us.
2014 Denali 289RK
2017 Nissan Titan XD Pro-4X CC.