Your exact process will depend somewhat on how the P/O winterized it. The most common method is as follows, more or less:
Drain Fresh, Grey, and Black Tanks
Drain Water Heater
Pour 10 +/- gallons of RV antifreeze in to Fresh tank
Turn Water Heater Bypass Valve to BYPASS
Run H + C faucets off water pump to pull antifreeze into lines, until it comes out the faucets.
Flush toilet until anti freeze comes out
Pour cup or two of antifreeze into each sink to fill P-trap
If there is an installed W&D, run wash cycle for 30 seconds or so to get antifreeze in to lines.
Remove the coach water filter element, and bypass the filter.
There may be other methods and steps, but this is basically the idea - you want to get antifreeze into all the lines leading from the fresh tank, and up to each faucet or toilet valve, and then into the P-trap below the sink. The small amount of 'fresh' that you might have left over in any of the lines should not be a problem.
To de-winterize, you need to drain any leftover A/F in the fresh tank out. Fill about 1/2 full or so, and run every line until the A/F is pretty much gone. This will also push the A/F out of the P-traps, but they're no big deal. Its the supply lines that you want cleaned out. Don't forget to add the new water filter element.
It may take a while to get the a/f smell out of the lines. Just keep running water thru them (you can use your city water connection if you want after a while, to save wear and tear on your pump. The check valve between the city connection and the water pump should be pretty close to both, so once you've run a couple of dozen gallons through with the pump, it shou ld be clean. If you want to you can also dump a couple of cups of bleach in the fresh tank along with 30 or 40 gallons and run each faucet until you smell the bleach, and then let the whole rig sit overnight. This will clean and disinfect as well. Don't forget to drain out most of that water too, or you'll have bleach for breakfast.
Lastly, un-bypass the water heater, and open the hot faucets, and let the hot tank fill. There are several good videos on YouTube on this process.
This is a lot of work. I did it a bunch of years in a row, and then last year I decided to try to 'dry' winterize process. Basically, you drain everything you can as usual, but rather than pumping A/F through the lines, you attach a special connector to your CITY water inlet that attaches to an air compressor line. Using about 40-50 PSI, you open each faucet in turn, and simply BLOW all the water out of the lines. Sinks, toilets, etc just open 'er up and stand back as the Old Faithful sprays out of the thing!
This method takes about 20 minutes, and the good part is, there's no de winterizing to worry about - just dump in some fresh water, some baking soda, and open the faucets. Once the air stops you should get pretty clean and fresh water out.
If this is all really confusing, your best bet is to look up the RV Geeks videos on YouTube - they're pretty good at covering everything.
EDIT - forgot about the fridge. IF you have the icemaker, and it was in use, you'll need to make a couple of dozen trays of ice to clean it out as well. There is also some sort of bypass mechanism for it, but I don't use ours, so I'm no help there. You may need to check the manual for your specific fridge.
Dewinterizing Excursion
Linear Mode
