With temps going to -20, it would be a gamble to only blow out the water with air IMO. The water lines are not positioned to 'gravity flow' one way or another. Blowing out the lines removed most of the water but any left in with gravitate to any of the 'low points' in the water lines presenting a possible freeze/burst location.
Remember, you also have to winterize the drain lines that contain "P" traps below the sinks/tub/shower that are filled with water...and drain the black/grey water tanks.
I have blown out the water lines with air but always followed up with RV antifreeze. The few $$ cost for 2 or 3 gallons of antifreeze is cheap insurance over replacing damaged lines or fixtures.
That said, I can see where it would be a hassle doing it two or three times a month in the situation you describe....as it would be with a site built house.
2012 Fun Finder XT 276 Toy Hauler & Harley Davidson Ultra Classic with Condor wheel chock
2014 Dodge Ram 2500 AT 4X4 Crew Cab 6.4 L Hemi, 373 gears, tow package, Rambox option, Revolver X2 tonneau cover, Equal-I-zer anti-sway hitch.