Sorry but WDH is for tongue weight and not truck cargo.
Set it up with the trailer loaded for camping and don't worry about the ATV, as you don't want that force back on the trailer. 
A lot of the water weight depends on the location of the tanks, but I don't worry about the weight difference. Set the WDH up for a full TT. 
If you find you have more sag than you like with the ATV loaded I would look into a set of airbags.  Use the airbags for cargo weight and WDH for tongue weight. Each has a different job/purpose.