We use bottled water for drinking as well. The water in the fresh water tank is used for showering and washing dishes. We don't normally fill the tank completely in order to keep the weight down, we travel with it about half full and add fresh water as we use it. Whenever we dump the grey and black water tanks I flush them out with whatever is left in the fresh tank so it's normally fairly empty.
I will dump it if there has been any sitting for a month or so between trips. Since we live out in the sticks, I just open the valve and let it run down the hill towards our septic tank. (I never dump the grey or black water anywhere except a dump station)