^^^^This.
I do not know your trailer size or tow vic however, I assume a dodge cummins just from your forum name (6bt and all...). Water is heavy and moves a lot if the tank is not baffled. Personally from your description I'd take between 10 and 20 gallons and a filter. Water for showers or dishes does not have to be filtered. Your in the PNW so water should not be hard to find and lighter is better almost 100% of the time.
My tow rig is an 80 series Landcruiser, trailer has a 5x10x5 box.
My other rig is a 6bt Excursion with 6 spd and dual disc clutch. manual t-case.
The excursion is nice in open areas, but hard to maneuver through dense forest roads.