Greg-
According to the wiring diagrams for your coach (link
here), the circuit that feeds the water heater comes off the third 120V branch breaker in the power center. It appears as if the breakers are horizontal in the power center, so it would be the third branch breaker from the top, which is also the fourth breaker from the top, counting the 120V 30A main breaker. It is a 20A breaker.