When should you winterize your trailer? I do it when I know I am done camping for the season. If you camp late into fall keep an eye out for freezing temperatures. If a glass of water would freeze outside then its time!
These are general Winterizing instructions- If you are unsure about what you are doing then I do recommend the winterizing of your trailer be done by an experienced RV Technician.
Drain all the water from the system by opening the drains on your fresh water tank, water heater, and any other valves in your water lines. Don;t worry about a small amount in your fresh water holding tank.
Open all faucets. When all water has drained from the system, turn your water pump on to expel any water remaining in the fresh water line.
Check that holding tanks are completely drained.
Next, close all taps, drains, etc., making sure that the city-water check valve is secure or plug the service port.
To put NON TOXIC RV ANTIFREEZE through the hot lines as well as the cold, you must by-pass the water heater. Some have a by-pass already on the trailer. If you don;t- consider installing one to save time and effort each year, If you have no bypass you must detach the hot and cold lines from the back of your water heater and join them together.
Next, we pour NON TOXIC RV ANTIFREEZE ( like, https://amzn.to/2RFki4Z) by removing the inlet hose on your pump and inserting it in the antifreeze container. If your inlet hose is not long enough, use another length of hose.
Double check that all the tap are closed. Next turn on the water pump go to the furthest tap and open it until you see the purple/pink colored antifreeze come through. Do the same with the hot tap, then go around your trailer and open each tap. Remember to do the toilet as well.
Shut off the pump and release the pressure by opening any tap.
Go outside your trailer and remove the screen at the city water depress the inner stem, stand back and wait for all the antifreeze to drain out.
The last step is to pour about a cup of the antifreeze down each drain.
Open the by-pass on the water heater or remove the temporary by-pass.
This site: http://www.rverscorner.com/winterize/winterizebook/index.htm has a good little manual for wintering your RV or trailer.