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.
An average freshwater tank holds 20-100 gallons of freshwater. A class A RV will hold the most water, averaging between 75-100 gallons. Class C RVs will hold 35-60 gallons, Class Bs will hold around 20-40, and fifth-wheel trailers hold about 60-80 gallons. Smaller trailers hold 40-60 gallons.