We have a whole section on winterization which is good reading.
How I winterize:
1) run faucets and water pump until freshwater tank is empty
2) drain water heater by removing the plug, and place valves in "bypass"
3) shut off water heater switches and tape a note in place not to use until "un-bypassed"
4) open the drain valve for supply tank to drain the last of the water, should be no more than a gallon or so
5) run the water pump briefly with a faucet open to get the last of the water out of the suction line and pump.
6) apply compressed air to the "city water" inlet being careful not to exceed 40PSI and open all faucets one at a time until no more water comes out, being sure to include the toilet, toilet sprayer, outside faucet, and drinking water faucet. Briefly open the two low point drains, one a time, last
7) Repeat step 6 to be sure all the supply plumbing is dry
8) Briefly put the air to the black tank washout fitting to get any water out of the backflow valve.
9) Pour about 2-3 cups of RV antifreeze into each sink drain and into the toilet.
10) go to dump station and dump waste tanks
11) Step on the toilet flush valve so the antifreeze in the bowl goes into the black tank where it will help keep the dump valve from freezing. Pour another 2-3 cups of antifreeze in the bowl to keep it sealed so there won't be any stink.
12) Pour another 2-3 cups of antifreeze into one of the sink drains so there's some antifreeze in the grey water tank
13) Remove and discard the drinking water filter cartridge
14) If time permits put the water heater plug back in, put the bypass valves back to their ordinary "service" position, and close the low point and fresh water tank drains so that the trailer will be ready to go in the spring.
People who don't have compressed air can substitute pumping RV antifreeze through the plumbing for step 6 but it's more complicated unless you have already modified your water pump to draw antifreeze in through the suction line.
That's it for the plumbing but a full winterization has other steps:
A) Remove the soap dispenser pumps and store someplace warm
B) Defrost and clean fridge and freezer, prop doors open
C) Remove any food items accessible to mice (not in glass or metal packages)
D) Remove any water based personal care items that might freeze, body wash, shampoo, shaving cream, some kinds of hair spray
E) Remove any water-based cleaning products that might freeze, bleach, windex, that kind of thing
F) Remove any food and beverage items subject to freezing damage, like pickles, beer, pop. The cooking oil, spices, and plastic wrap can stay in the galley
G) After a cold winter the batteries in the smoke alarm, doorbell, and any portable items (weather radio, alarm clock, kitchen timer, meat thermometer, flashlights) should be replaced. Might be prudent to pull them out now in case they leak.
H) Top off electrolyte in the 12v
batteries and be sure they are fully charged, depending on your situation and what kind of converter you have it may make sense to leave the trailer plugged in over the winter to keep them charged
I) Shut off propane at the tank valves