I have my trailer on a seasonal lot. When the weather starts to get too cold I drain the hot water tank and open all of the taps/shower and blow out the lines with my air compressor. Then I'll use the water pump to put in a couple of gallons of rv anti-freeze and turn the taps off individually when it runs pink (Don't forget the toilet). A cup down each trap and I'm done. Once I've done that I don't use the water system. I'll use the toilet a bit in the fall if I have to but as soon as it's cold enough I line the bowl with a bag and out it goes to freeze over night. I use my trailer during the winter and don't have any problem.
Unless you fill the hot water tank with antifreeze, it flushes out of the lines very quickly in the spring. If you forget to drain the hot water tank and you fill it with antifreeze then it'll be in the lines for a LONG time if you don't drain it out.
I should add, don't dump the antifreeze into your water tank to flush your lines. Take the hose off of the tank that supplies the water pump with water and add the antifreeze from there. That way you only have antifreeze in your lines and it'll flush out easy.
