Farris, good find on the 21'er and welcome. Let's get to your question. Without having or installing a secondary suction hose, the only way I can see to empty your pipes is to pressurize (with an air compressor) at about 35 psi at the discharge side of your 12 volt pump. Blowing out one faucet at a time including the outside shower and your toilet. When your ready to do the hot water tank, remove the drain plug and blow the air thru it. Then take a cup or two of the antifreeze down each drain to fill the "P" traps.
PS: leave the HWT drain plug out. This should give you enough time to alter you pump system.. Also, drain all holding tanks, and leave those drains open as well.
I have a place in Andrews, NC near the Tenn boarder, I see a prediction of 24 degrees!!
2703 Tow Lite
Bonita Springs, Fl. &