I live in Minnesota-- We have some pretty extremely cold temps here (sometimes -20 to -30 F). I've followed this same procedure now for 12 years in various trailers and motor homes that I've owned, and have never had a problem (and I don't air compress the lines).
Drain and flush the fresh and black water tanks, disconnect city water (make sure your connecting fresh water hose is put away empty); add around two gallons of antifreeze to the fresh water tank (making sure that the drain is closed). Bypass, then drain the hot water heater, turn on the water pump, run each water faucet (sinks, bath tub, shower, outside shower, toilet) separately until the pink antifreeze comes out-- when doing this with the sink or bathtub/shower, run it enough so that you're sure that the antifreeze has filled the drain traps as well; run the toilet, get some antifreeze in the black water tank, leave some antifreeze at the bottom of the toilet. Make sure you run the outside shower until antifreeze comes out, as well.
And you're set.
If you really feel you must blow out the lines with an air compressor, by all means, knock yourself out; but in my experience, it is unnecessary.
BTW-- if you look hard enough, there are brands of Marine/RV antifreeze out there that are non-toxic-- they have worked as well as any of the more conventional brands.