A perfectly loaded and designed TT shouldn't need sway control. Your receivers rating would be the only limit. If the TT tows fine IMO I'd try it. Our last TT had the axles to far forward making the TT prone to wiggle. Never bad sway but it wanted to wander. And that's using an EQ with built in 4pt sway control. Our TT before that had the axles reward. Never needed any sway control and frankly never needed any WD. Used it though because of the receiver rating.
I highly doubt 900lbs would drop the rear of the 3500 more than an inch. And I know for a fact that the front will only lift about 1/4 - 1/2" That Cummins is pretty heavy and it takes a lot of weight to raise the front of the truck.