Joe,
Where would you mount a tank and how much water or liquid would you use? A 
gallon of liquid weighs approximately 8 pounds, so any significant gain would 
take a lot of water/volume. Also, think about a very heavy water projectile 
should it come loose while trailering or in a panic stop situation.
I think you are further ahead by doing what we already know to put weight on 
the tongue.

Rummy