Something that I have been doing for many years, is there is a small adapter made, that screws into the fresh water intake side, (male hose bib), that is adapted to a common schrader valve. Open all the faucets inside, and gently purge it with air.
Now before anybody gets excited, here is the usual disclaimer:
DO NOT put 120 lbs. of air pressure into your plumbing lines, unless you just have a desire to see what it looks like to launch a water fixture through the cabin roof. One of those punky little 12 volt models like you get at Auto Shack, (or whatever those chinese auto part stores are called), and put 5 or 10 pounds of pressure on it slowly. I have never busted a line, or had one freeze.
I also don't live in the extremes of the North either. I'm sure there is some residual water left in the system, but with 98% of it blown out, it has room to expand if it freezes.
And I've never had a funky aftertaste from old air in the lines come springtime..