Some systems have 3 valves on the tank side of the heater. You may have to find them by removing an access panel on the inside of your RV. Two normally open, to allow cold water in and hot out. One "bypass" closed. In winterizing, after blowing out all the other lines, the last thing is to pull the tank plug to blow/drain anything remaining in the heater.
If you are also going to use antifreeze, you close the hot/cold valves, and open the bypass. Then use the water pump to push the antifreeze through the entire system. (it too has valves to stop taking the water from the tank, and instead pull from a short hose that you stick into a bottle of antifreeze - usually found right near where the pump itself is mounted). The heater bypass is done so that you don't have to fill up the heater with 3-6 gallons of antifreeze before it begins to get to the rest of the plumbing!
My fifth wheel needs about 2 gal of antifreeze to do everything, and I get a little liberal on what I pour down the drains.
Takes me about an hour, but could easily be done a lot faster.
The dealer charges $145 to do it!
To recap, in terms of my method and experience with my unit:
(And, I should note, I do both as a belt and suspenders thing because it gets REALLY cold in Minnesota!)
All water and holding tanks should be completely empty to start. Run your water pump as needed to drain the water tank.
You can open the gray water valves, as you will be only dumping clean water on the ground, and perhaps a little antifreeze, which is safe and completely non-toxic. Leave the black tank closed.
Unhook shower lines and let hang.
Hook compressor up to city water outlet, using adaptor noted in another reply. Set to 40 lb (Jayco recommended). Start air flow.
Progressively open each faucet, both hot and cold, until just air comes out. Do not forget showers, toilets and any outside faucets. Open "Low Point" drains, and again make sure only air comes out. It may surprise you on how many repeats of the above you need before you get all the water out.
Turn off air for a minute. Open a faucet to relieve pressure. Pull main drain plug from the heater. Turn on air again, and blow out any remaining water in the heater.
If not using antifreeze, you're done - go to the drains (following).
If using antifreeze, set the heater valves to bypass the heater. At the pump turn off the water line from the water tank, and turn on the suction hose, put it in a bottle of antifreeze. Run all taps until pink stuff comes out. Include toilet, showers (with hoses detached), outside taps and low point drains.
Drains - pour antifreeze down drains - about a cup, more if you haven't already put some down by running it through the taps. Don't forget shower and some in the unflushed toilet.
Close gray water valves.
Reset the valves for the water pump to be able to draw from the water tank again. Leave the heater bypassed.
Done!
You can get by not using a compressor at all, and just pump antifreeze through everything. It will probably take more to ensure that you've replaced all the water with antifreeze. Still do the heater bypass thing, but you'll be letting it drain naturally, rather than forcing anything out with air pressure).
In the spring - don't forget to reset the heater bypass valves! Otherwise, just run all water a lot to flush out the pink stuff.
