The fresh water tank in your RV supplies the water that comes through your faucets and through your fridge in-door water and ice maker (if you have a fridge with that option). Some tanks are larger than others. Depending on the length of your trip, tank size, and location you may find yourself needing more water in your fresh water tank. For example: boondocking (camping without any hookups, usually in a remote location) for a long period of time will mean you need to conserve water or re-fill the fresh water tank.
So what do you do when the fresh water tank in your RV is empty? Do you just fill it from anywhere? Can you pull up to a fresh water stream and fill up with that water? What about water that has not been deemed potable at a campsite? Can you filter the water and make it safe enough to use?