Hayes Sway Master Elec Sway Control. Does it really Work?

oops... I didn't read the "get me close to 600"... then you need to add 400... :-)
I think your tongue is way to light but your strategy of incremental adds will likely find the solution. Your best bet is load up with your "normal gear" along with supplies (ie water, propane, cords, etc) and head to a scale and get a more accurate weight scenario and go from there.