I have always understood that the heater capacity is the number quoted. Often the fresh water capacity is stated as the size of the fresh water tank plus the size of the water heater.
Please let us know after you take cpaulsen's advice.
Class C, 2004/5 Four Winds Dutchman Express 28A, Chevy chassis
2010 Subaru Impreza Sedan
Camped in 45 states, 7 Provinces and 1 Territory