I use both air and antifreeze to winterize.
Drain all water from the system: fresh water tank, water heater, low point grain lines, and valve near water heater, with all valves open, hold open toilet fill line for a few seconds
Using an adapter at the city water line - blow out the lines, opening and closing valves and drain lines. you need a compressor with a tank to do this. IMPORTANT -- NEVER EXCEED 60 PSI, and always have one valve open. The water heater is bypassed for this.
Drain black and gray tanks Flush black tank several times on your last outing, leave dry.
Purge system with antifreeze - using water pump bypass input hose. Starting with high point (shower) open up one valve at a time (hot and cold), until antifreeze runs through. Pour 1/2 cup into each trap. Pour 1/2 gallon of antifreeze into black tank.
I have been doing this method for 25 years, and have never had a problem. It is my opinion that the valves and piping system is in better shape with antifreeze in the system, rather than dry -- lubrication etc.
Tom & Lori
2013 27 FB Classic Limited
2016 Nissan XD Turbo Diesel
WBCCI 03288 TAC PA-17