I usually open the tank drains after I have started my truck to leave. That allows the water to start draining as I drive along. Chances are that if I using water from my tank, I don't have a sewer hookup, so I am going to the dump station. Unless I really overestimated my water needs for the trip, I am out of water by the time I've waited in the dump line and dump my tanks.
I would never store water for long periods of time (even a couple weeks) in my freshwater tank. That is a breeding ground for bacteria and disease. Sanitizing the tanks initially is a good start, but it is not a sealed system and it is impossible to completely clean and sanitize the entire tank.