One of my seasonal trailer's water heater drain plug is stuck and will not open. I blow out the lines with a compressor. I then close all drains and faucets. Then I pour three gallons of RV anti-freeze into the empty fresh water tank. I then add three gallons of water. (RV antifreeze is good to -50 degrees. An RV dealer owner told me by diluting it with water, it is still good to -25 degrees. (If it gets colder where you are, do not dilute).
I then turn on my water pump and let it build pressure. I then go to the farthest faucet from the fresh water tank and turn on the hot tap. Once pink AF appears, I shut it of and go to the 2nd farthest faucet and repeat and then the last. Once all of the hot water faucets are running antifreeze, I repeat the process with the cold water.
Been doing this off & on since 1983 and have yet to have any problems.
On the large parked 30 footer, since the HW tank has a drain, I blow out all of the lines. I then add antifreeze to the sink & bath tub pea traps and put some in the black tank and a couple of cups in the toilet. I also leave the toilet seat up to avoid any growth of mold (don't ask how I know).
These two trailer are in a part of Washington state that can get below zero at times.
2015 White Hawk 20MRB
2011 F150 Non EcoBoost (5.0)