The way I do it is as follows:
1) When I use RV antifreeze, I leave the water heater on bypass until all the RV antifreeze is out of the water system.
2) Open the temperature/pressure relief (TP) valve near the top of the water heater. It is a lever located on a brass valve and is accessed from the outside door where you access the water heater.
3) Locate your water heater bypass valves and turn them to where water enters the water heater (Not bypassed).
4) When water starts running out the temperature/pressure relief (TP) valve, close the valve. Since this valve is slightly below the top of the water heater tank, there will be an air pocket above the valve.
As an aside, if during normal operation, the air pocket is lost (a sign of this is leaking from the pressure/temperature valve), shut off the water supply (pump or valve supplying water to your coach), open a hot water faucet, then open the temperature/pressure relief valve and allow the water to run out of the valve. CAUTION: if the water heater has been operating, the water from the TP valve will be HOT!! From a safety standpoint, this procedure is best done with a cold water heater, but with caution, can be done when the water is hot.
Hope this helps.