An RV is somewhat self-sufficient. Your RV has an on-board fresh water supply as well as holding tanks to capture the gray water from the sinks and black water from the toilet. It’s important for the RV owner to know how much fresh water is left as well as how much room is left in the holding tanks. To monitor these levels RV manufacturers include a tank monitoring system. These systems generally consist of probes placed at 1/3 tank level intervals. When the fluid in the tank reaches a given probe it creates a circuit between it and the bottom tank probe and lights up 
