I am not familiar with your specific unit, but here are some ideas:
Are you referring to the hot water heater bypass? If so, it will probably be next to the hot water heater. Make sure the cold water input to the water heater is off and make sure you release the pressure valve before opening the drain.
If you are referring to the valve for adding antifreeze, that should be in the compartment where you attach your water hose. Be sure to run some antifreeze through your pump into the fresh water tank and your outside shower if you have one.
Don and Michele
3 Adult Children - 5 Grandchildren
Present campsite - future homesite