I lived in southern Indiana where I owned 2- TT and a MH. I always used the compressed air method. Some of the winters the outside temps got down to -20.
Never had a problem. When I did the air, I tried to make sure all the water was out of the lines, toilet, shower, low water lines, etc. I did pour antifreeze into each of the traps - bathroom sink, kitchen sink and shower. I left a cup of antifreeze in the toilet. I have never put antifreeze into the water lines. So I vote compressed air only into the lines.
2015 Flagstaff 26VFKS
TV - 2014 F150 Crew Cab 5.0L
WDH - 1,200 Equalizer