Can I just add that I use a compressor to blow mine out, I unscrew the shower head and connect the compressor to that. Set tap to cold, open all other taps on cold and start compressor, then do the same on the hot side.
I use one of these http://www.keepfloeing.com/products-motorhome.html
together with a 12v tyre pump. I was surprised how much more water comes out, even after a standard drain down I can usually purge an extra litre or more. Saves a lot of sucking and blowing.