HWH by-pass not working

[RVingUSA]

My husband has winterized our 2011 3400 twice now and antifreeze keeps getting into the hot water heater. Any ideas why this might be happening?

[1Happycamper]

Where the hot water comes out of the tank at the top there should be a check valve. The valve is to let water flow out of the tank only. If it is stuck open it will let antifreeze flow into the tank. Changing the check valve is easy.