I do not have a bypass circuit for my gas water heater on my 73 Argosy but after a bit of head scratching I have discovered a work around. The shower head lockout switch. I drain the system as good as possible and shut off the valves on the heater inlet and outlet lines. I drain the heater as normal. I put 4 gallons of RV antifreeze into the empty fresh water tank and start the pump. Open the cold water taps one at a time until the pink juice comes out, shower head last. Do not overlook the toilet and the rinse hose as well as the shore water inlet and the 2 drain valves under the back.
Now, with the shower shut off valve slid over to stop, turn the shower hot tap on while leaving the cold on. If you have a single mixer valve set it in the middle. The pink juice will now enter the hot water lines from the back heading forward. Open the bathroom vanity hot valve until you get pink juice and close it. Go to the kitchen and turn on the hot water until you get pink juice there and presto you're done. All lines are protected. Don't forget to protect the black and grey tanks and leave some pink juice in the toilet bowl.