Did you upgrade the trailer frame, springs, axle, tires, and wheels too? That's a lot of added cargo capacity for a 26 foot trailer. I'd expect you're over the limit even before adding any water, propane, or a single item of clothing.
And carrying capacity is generally based on an evenly distributed load, which yours definitely is not.
Adding more tongue weight MAY reduce the sway, but maybe not. That's a huge point source on a long lever way back at the rear, so any little rocking there just gets magnified as it projects forward.
Maybe a Hensley might help?
My advice, given the amount of money you've probably already invested in this, is drive VERY SLOWLY (always UNDER the speed limit), be extremely careful with turns and sudden moves, don't drive in windy conditions, and always keep considerable distance from other vehicles.