Recreational vehicles usually come with three water tanks. One is for fresh, potable water and is usually the largest of the three. Grey water tanks are usually the second largest and are for water from sinks, showers or dishwashers. Black water tanks are the smallest and are for human waste.
Most RV water tanks are made from polyurethane and other plastics because they do not corrode, are easy to disinfect and are lightweight to improve fuel economy. Fresh water tanks need to be filled from a fresh water source, while the grey and black water tanks are emptied. They also must be disinfected with a bleach solution after every trip.