It is pretty simple to winterize the water system in a trailer. I don't know what your setup is but I'll tell you how I do mine.
1. If I know I am done camping for the season I drain the black tank. I fill the tank twice and drain. I have a black tank flush system that I use after draining the tank leaving the tank valve open until I am done.
2. I drain the grey water tanks, and fresh water tank if I have water in it. I open the low point drain valves and open faucet. I remove the water heater plug and lift the pressure relief valve to drain the water heater.
3. I close the low drain valves. My trailer has a lever to bypass the water heater and another valve to by pass the fresh water tank. I place both on the winterize side. I have a 4.5 gallon container I use to use on previously owned boats. You don't need 4 gallons of antifreeze but I prefer more than not enough. My trailer also has a plastic tube that I put in the container of antifreeze. I also have an outside shower so I connect the hose from that to the city water connection.
4. I turn the pump on and turn on faucets, one at a time until I see pink antifreeze coming out. I do this for the kitchen sink, bathroom sink, and shower. I then flush the toilet, mine is porcelain, until I see pink.
5. I turn the pump off and open a hot and cold on one of the faucets, go back outside and open the low point drains. I don't for the fresh water tank because I don't put anything in it. When I see pink I close the low point valves and the hot and cold on the faucet.
6. I take what is left of the antifreeze and pour a little down each drain. I pour some down the toilet into the black tank. I leave a little in the toilet to keep rubber seal from drying out. Since the toilet is porcelain I don't need to worry about the antifreeze staining the bowl.
The reason I hook the outdoor shower hose to the city water hookup is so I don't forget to winterize that too. It is used to winterize everything.
I takes maybe 20 minutes and around $10 of antifreeze.