Rockwood/Flagstaff - travel with full fresh water tank?

I'd make the argument that 2/3 full is worse than full. If the tank is full, it doesn't slosh so it's dead weight. A sloshing tank will act like a slide hammer on the tank hangers.

I found this out the hard way when I was a kid driving a farm truck with a water tank on the bed. It was half full I stopped at a stop sign, and the sloshing water bounced me out into traffic.