Turn off water heater as well during winter?
Ideally..


Turn the power/gas off to the water heater.
Turn off the water to the building.
Remove the water from the entire building, including the water heater, toilet tanks and water supply lines.
Pour RV antifreeze into every waste trap, including the toilet bowls.


The building is now winterized and your safe even if the heat goes off.
If the heat goes off and you haven't removed all the water, you will probably pay for it.