I have been blowing out our lines for years and have had a couple of problems, not with the blowing out, but with forgetting to open all the lines. I just hook up my compressor and blow out the faucets, toilet and other places where water comes out. Then I put pink RV antifreeze in the P traps. Don't forget the open your low point drains as well. Also, your outside shower, your outside sink and, maybe your outside black tank rinse connection. I messed up two years ago and think that I left water in that area and caused it to crack, thereby letting water run into the FW rather than into the black water waste tank. Betcha I don't forget this year, once I get it fixed.