I store my MH in Florida, and the worst recent Winter had temps down in the low 20's for a few nights, then above freezing in the days. Even in this "mild" freezing, I don't trust air only. With antifreeze, you can be sure that the water is purged by watching each outlet turn color. There is no "proof" with air.
$8 worth of antifreeze once a year to protect thousands of $$ of equipment is the cheapest insurance that I can think of.
2008 Itasca Meridian 37H & 2015 Flagstaff T12RBST
2011 & 2012 Len & Pat's "One lap of America"
27K miles & 41 states in 13 months - Woo Woo
Yellowstone Lake 6-1-2012