Air bags, or heavier rear springs, do not re-distribute weight, and that is why steering will be floaty.
I used a weight distribution hitch on the 100 series, and am now using it on the 300. On a recent trip the ball weight was 330 kg and as at all times using the WDH there is no difference in the feeling of the steering from when not towing. The steering is responsive and positive.
There is an unfortunate lack of understanding as to how WDHs work.