You should NEVER
allow RV antifreeze to go into the water heater. There is a bypass valve near the water heater that you should set so liquid from the fresh water tank will bypass the water heater.
You can either completely drain the plumbing system and maybe blow it out as Frank mentioned, or you can run RV antifreeze throughout the plumbing system to prevent freeze-up. If you do a good job of draining the system, there won't be enough water left in it to freeze. Or if you do a good job of winterizing the plumbing system, it shouldn't freeze. I usually run the RV antifreeze with the following steps:
A good job of winterizing involves:
1] Drain all the water drains under the trailer, including the fresh water tank and the water heater. Open all faucets and flush the potty so any water in the lines will drain. Then drain the grey and black water tanks. Last step of draining is to close all the drain valves.
2] Stop water from flowing through the water heater. There should be a bypass valve near the water heater to do this.
3. Pour fresh RV antifreeze into the fresh water tank. Mine requires at least two gallons to have enough to do the next step without pumping air.
4. Close all the faucets. Turn on the water pump. Run each faucet (both hot and cold valves) until you see the pink antifreeze come out. Include the bathtub/shower valves. Flush the pottie until you see the pink antifreeze come out into the comode.
De-winterizing includes draining and flushing the fresh water tank and filling it with fresh water. Run all the faucets and pottie until the water runs clear. The last step is to change the bypass valve near the water heater so fresh water will flow through the water heater.
Yes, the above is a copy from another post I made a few minutes ago, in case you don't read all the threads in this forum.