The size of the holding tank you need will depend on how much water you use per day and how much wastewater goes down your drains.
When buying an RV, it will say something like water tank size is 65-25-30. This means that your freshwater tank is going to hold 65 gallons, your grey water tank can fill up to 25 gallons, and your black tank should stay under 30 gallons.
Fresh Water Tank
The first and generally the largest holding tank is the fresh water tank. It is usually designed to store and provide enough fresh water to last a little over a week of average use.
The size of the freshwater tank in an RV varies with the design and type of camper, but the tank size is generally designed to hold more than the GRAY and BLACK water tanks.
A pop-up camper might have a tank in the 10 to 20-gallon range while a large Class A motorhome or coach can have a freshwater tank with a capacity of 100 gallons or even more.
Gray Water Tank
The size of a gray water tank varies depending on the type of camper or RV. Generally speaking, it is designed to carry up to one week of waste, assuming there are only two people using it.
Black Water Tank
A black water tank is designed to hold two individual's excrement and toilet paper for about one week.