Need more tongue weight. Will a water tank up front do?

Are you looking for 10% tongue? That is 1500 or 400 more pounds needed. That would be 60 gallon tank minimum.
Are they using a weight distribution and sway control hitch? Or is this a pintle hook?
Consider adding shocks and/or a sway bar.

What is pulling? Maybe hire a pro with a bigger truck when it moves.