Just to reiterate the key with air pressure is for it to be high enough to really blow out all the water. My dealer winterizes trailers without antifreeze but the key is the pressure. My little 12 volt
can't generate the amount of pressure necessary so while I use it to displace the large volumes of water, I follow up with RV antifreeze to make sure. If you don't blow the lines out well, any residual water left in the system will drain back to a low point, and if enough gathers it becomes freeze time.
The other thing to remember for those of you using air only is to take care of the drain traps and get some antifreeze into those. One trick I have used is to reverse my shop vac and actually blow the water out of the traps, then add a small amount of antifreeze to keep the traps from venting sewer gas...and don't forget those minor areas when you blow out lines like the toilet rinse sprayer, the kitchen sink sprayer, and for some of you the outside water outlet.