3 Methods
WET: come spring you have to flush all that******out of your fresh water lines. Major job. I did this the first winter NEVER AGAIN
DRY: Do not blow just one time, Blow several with a few minutes between blows, Come spring, nothing to flush out, and if air freezes, the RV won't be on your list of things to worry about (just one worry at that poing.. UP or Down). Use antifreeze in the traps and toilets.. Flushing them come spring is no tribble at all. (Yes, David Gerrold is on my Facebook friends list).
NOTE: Some folks yammer about "That little bit of water that clings to the sides of the lines" First, it won't bother the pex tubing (only the hard plastic fittings and it won't settle in the fittings) second. The multiple blows gets rid of nearly all of it. I never had a problem the winters (Several) I used the dry method.
S... What I use (Drive south and forget the above methods)