Here is the steps I use and my coach has survived below zero temperatures outside (though now it is inside).
1) Use and air compressor and buy one of these:
2) Attach item above to the city water connector.
3) Opening all the faucets and all the low end valves, turn your compressor to approx 45-50 psi and attach it to the blow by plug above (that is at this time attached to your city water connection.
4)After running for a few minutes, go ahead and close each open valve and faucet.
5)Turn the pressure down to about 40psi and open one facuet at a time, blowing out first all the ones in the coach. Once done, then open the low end valves and empty both hot and cold.
6) Open the water heater up, and release any water that is in the tank.
7) Dump the remaining water in the fresh, gray and black tanks.
7.5) If you go no further, then simply put RV antifreeze in the sink traps and a bit into the black, fresh and gray tanks.
You have now sucessfully purged most of the remaining water in the lines and water heater and done the basics. Most folks see this next part as optional, but it's cheap and takes only about 30 minutes more as added insurance.
8) Where your pump connects to the fresh water tank, disconnect it. At your local harware store, buy a fitting that will connect to the pump intake to a hose that you will also buy at the hardware store. Connect the fitting to the pump, attach your hose to the other end of the fitting and place the hose into a gallon of antifreeze.
9) Blow some antifreeze out the lower end valves(if your Airstream has them). When you open one, the pump goes on, when you close them the pump will go off. Open both the hot and cold and rear valve near the water heater. You only need to see pink fluid come out, and it will very soon, so you don't need to let it run long for each of the three (again if your Airstream even has them.
10) Next run the pump (making sure that the gallon of RV fluid still has some in it) by turning on each faucet, hot then cold (or cold then hot) one at a time. Allow the fluid coming out to be pink in color before move to the next faucet or water using fixutre.
11) Pour about a half gallon to a gallon (depending on tank size..larger using more) into the fresh tank. Why? Well, even after the tank is empty, you don't really get ALL the water out, so I pour about a gallon into my 39 gallon fresh tank. Overkill? Probobly, but in my opinion an ounce of prevention is worth 100lbs of cure.
12) Take some of your remaining RV antifreeze and pour some in each of the sink, shower and/or bathtub drain traps, and a bit for the black tank. Remove your hose attachment from the pump intake and replace the fresh tank pipe to the pump. **remember to check for leaks in the spring to make sure you connect the fresh pipe to the pump correctly **
You have now fully winterized the plumbing in your Airstream using the Tim Allen method. After your second time around you should be able to do all this in about an hour and use about 3 gallons of RV antifreeze, maybe 4 if you forget to close the low end valves (which I have never done).
If you can give it a wash and a walbernize, inflate the tires to the proper max pressure, yer done, provied you have taken all the provisions out of the camper that are perishable, can freeze (liquids...soda, water, beer, etc) or be found and open by any critters that may get inside. Also prop your fridge open and put a box of baking soda in the freezer and the fridge as you might in your home fridge and freezer.
In the spring, connect the city water to your garden hose (fresh water approved hose) and open all the valves talked about above and let the water run through the system, fresh tank and water heater for about 20-30 minutes. After you are sure the fresh tank is flushed, put about 3 ounces of bleach into the tank, fill it up open the faucets to let the mixture get into the pipes and let it sit for about 3-4 hours. Reflush everything. If you have a bit of clorine taste after you flush everything, you can add a bit of baking soda to the fresh tank (very little mind you) and most of the taste should be gone.
Two side notes: I switched from the conventional RV antifreeze to one sold at Ace Hardware that is mint flavored RV antifreeze. Not as nasty if you have some residue left behind as the conventional stuff. The other thing is to take out any of the water filters you have. Our Safari has one in the galley sink built into the faucet. Make sure to take it out before putting RV antifreeze through it or you will never get the taste out of the filter.....ask me how I know.