Can you put more water in the tank after it overflowed? I know I am able to get more water in after the first "overflow." After the first overflow I reduce water pressure and it runs for some time.
Also, RV makers sometimes, if not often, include the capacity of the hot water tank when stating fresh water capacity. I believe in your case it is a 12 gallon hot water heater so if they counted that it would take capacity down to 56 gallons. (How you are ever supposed to use the water in hot water tank, without draining it, is beyond me.)
Mike