Here in Tucson we can see temps down into the twenties a few times just about every winter. It only gets that cold for a few hours and not very often. Once we start to get sub 32 degree nights I open the low point valves and let all of the water drain from the system. This is even more effective if you drive a few miles as that helps to clear the lines. Then I add anti freeze {the pink RV stuff}, to the shower and sink drains, and both the black and gray tanks.
This leaves the rig ready to travel with nothing further to be done, other than adding more fresh water to the tank. Been "winterizing" in this fashion for 7 years and so far so good.
:C
