Never worried about emptying the tank till I read a few comments here, better to be safe than sorry I thought.
Then I found the tank doesn't have a bung for drainage, so, after removing a couple of pipes I managed to drain it, mind you, it had already sat for 5 / 6 weeks after the last trip. Nothing wrong with the water, but left it empty with the cap loose so it could air dry. Next trip, all packed up and doing the final check before departing, remembered I hadn't filled the water tank, so in with the garden hose (ours doesn't flavour the water) and proceeded to fill the tank. Several air locks later (over an hour) it was full! Turned a late afternoon arrival at camp into an overnight beside a main (think trucks all night) road! I have never drained the tank again!