RV holding tanks refer to two types of water tanks. The first holds what is called gray water which is the waste water that accumulates from the use of your sinks, shower and/or tub if you have one. This tank will never have human waste in it. There is a special tank for that as well. The second holding tank is for “black water”, the human waste from your RV toilet. All RVs have different shape and size needs for holding tanks, some have more than three (more gray water tanks), but there will always be three, three tanks for your RV in total; the fresh water tank, the gray water tank, and the black water tank.