Water weighs 8.3 pounds per gallon so you've got almost 600 pounds of cargo supported almost exclusively by the rear suspension when the tank is full and you add in the weight of the other stuff that you mentioned. The Xterra (or any other small to mid-size SUV for that matter) is not a good choice for your needs. It's not realistic to expect your truck to carry that load over 4-6 inch potholes without bottoming out.

If you absolutely have to use it this way you can explore the installation of overload springs. They will add firmness to the ride while loaded but shouldn't make the ride any harsher while empty - but I wouldn't expect tremendous improvement and I'm not sure this will completely resolve the problem. You can also consider moving the tank forward so the weight is being distributed over the front and rear suspensions - but you'd have to fold the rear seats down to do it.

A better alternative is to move up to a larger pickup truck for this kind of use - or mounting the tank in a trailer.