Close the hot and cold Ball Valves and open the by-pass valve when you are flushing the system with antifreeze.
The Water heater will take a great deal of RV Antifreeze if you are using RV Antifreeze to flush the water system. Its almost impossible to flush it all out of the heater if you do get antifreeze in there. The water to the hot water system needs to go through the water heater. Putting in By-Pass valves,will allow you to flush the system very quickly. There are kits that you can buy from the RV center but I find that they corrode and seize up regularly. It was easy fro me to put the valves in while I was repairing the plumbing and these ball valves do not corrode as quickly.
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