The RV fresh water tank holds clean water in order to provide you with running water while off grid. When using the RV sinks, shower or toilet off grid, the water is pumped from the fresh water tank. The RV fresh water tank should always be filled with potable water. When an RV is connected to a campground water hookup (aka city water) the fresh water tank is bypassed and all incoming water comes directly from the campground water spigot. Always use a dedicated drinking water safe hose for fresh water. Never use your black water hose to fill your fresh water tank or connect to city water!