Brian,
If your system is similar to the Hurricane unit, the heat exchanger is employed to provide heat from the engine coolant when the engine is running. When parked, the water heater is used. Does your lack of hot water occur in both cases? Assuming that the unit is plumbed so the output of the heat exchanger flows through the water heater before feeding the coach output(s), you should have hot water at the water heater if I understand the system correctly.
Steve