If it's a Suburban WH, take the anode rod out and open the manual override on the pressure relief valve. The water will empty itself. If it 's an Atwood, which doesn't need an anode rod, there's a drain plug at the lower edge of the tank (about 6 o'clock position.) Same procedure.
Having done that, look down below the tank inside the rig. On mine, there's one hand valve. When turned off, no water goes to the WH.
Personally, I choose to blow all the lines free of water using an air compressor. WalMart and other stores selling camping stuff will have an adapter that has a tire-valve sized stub in the middle of a plug that goes in the female end of a water hose. hook that up to the city water inlet, connect your compressor chuck to it and get the output air pressure to about 40 psi.
Go round and open each faucet until it blows air out (no more water) closing each one after it has been blown. Don't forget the toilet and the outside shower (if you have one). Finally open all the faucets and verify there's no water coming out of any of them when the air is blowing.
All you need to do then is pour a bit of the "pink stuff" down each sink to prevent freezing up of the P-traps, and you're done. On my rig, the whole process takes less than 45 minutes.
Note that, where we live, we don't get the horrible minus temperatures that other places get. It's rare to see an overnight low much less than 24F and almost every day gets into the mid 30's for a high.
Frank Damp -Anacortes, WA,(DW- Eileen)
ex-pat Brits (1968) and now ex-RVers, as of 08 Dec 14.