Drain fresh water tank, leave valve open. A small amount, around a few ounces may settle in a low area, but when it freezes it will not cause a problem.
And do NOT add RV coolant to the fresh water tank. Take forever to flush; or so I was told.
In the spring you may also want to sanitize the fresh water tank. I use a cup of household bleach for my 20 gallon water tank every spring. I let it sit for about six hours, then run for a few minutes through the water lines and heater, then drain both tank and heater.
Then I flush both the water tank and heater a few times and never had a bad taste or a bleach taste in the water.
Trailer: Lifted 228BH, heavy duty springs and Yokohama tires
TV: 2016 GMC Sierra Z71 4x4 CC, SLT
Spare TV: Two Alaskan Malamutes
Living somewhere in ID; previously lived in Moab UT; previous to that, don't ask!