Did you check for an antifreeze suction line. Most trailers now have a suction line in the UDC that allows wimterizing by just turning valves properly.
I to been using air for 20 years and never paid anything for winterizing.
Barbara and Laurent, Hartland Big Country 3500RL. 39 ft long and 15500 GVW.
2005 Ford F250 SD, XL F250 4x4, Long Box, 6.0L Diesel, 6 Speed Stick, Hypertech Max Energy for Fuel mileage of 21 MPusG empty, 12.6 MPusG pulling the BC. ScangaugeII for display..