I always leave at least a little in case I need to use the sink while parked. It is better to keep water in the system so things won't dry up.
I usually add a 1/2 cup of clorox bleech with a full tank on my first run to make sure nothing is growing inside the tank or piping. Then I dump the tank fully and add enough for the road trip time.
I usually don't drink the tank water. I'd rather drink the campground water or bottled. The rank is good for washing and flushing.