The amount of water your RV fresh water tank will hold depends on the size of the RV.
Smaller RV's and Travel trailers hold from 20 to 40 Gallons of fresh water in the fresh water tank and larger RV's and Travel Trailers can hold from 50 to 100 gallons of fresh water in the water tanks.
A Class C RV will hold from 35-60 gallons of fresh water while A Class B RV will hold around 20-40 gallons of fresh water.