Originally Posted by PTHutch
I don't worry about frost warning.
What you need to worry about if the temperature drops below freezing for the overnight, and for a length of time that it will create a hard freeze (i.e., freeze water).
That's when you need to follow the procedure that Jon indicated above to insure there are no problems.
Temps need to get down below freezing (32 degrees F.) and stay there before water will expand. When it does, it expands by approximately 9%. This expansion is what breaks things if the water does not have some place to expand into.
Leaving a quart or two of water in the bottom of your WH is very unlikely to damage the tank over winter. The water expands in the easiest direction; ie. up into 4/ 1/2 gallons of air.
Likewise, draining your fresh water tank and leaving the drain spigot open and blowing out the line between the tank and the pump (and the pump) means that any residual water in the fresh water tank won't be able to damage the tank or the lines. It simply expands into 25 gallons of air. As long as the drain line and the line from the tank to the pump are clear, they can't be damaged either.
Blowing out the S traps of your drains accomplishes the same thing. Most folks like to put a bit of RV pink antifreeze in the drains to prevent odors from backing up into the trailer and to doubly protect the S trap from freezing and breaking.
General rule is the smaller the space that holds the water, the easier it is to break when it freezes. The relatively tiny water valves in your toilets are particularly susceptible to this. (DO NOT ask me how I know....
Same for pipes in the trailer. The PEX tubing that they use these days is more robust, but nobody wants to test it in their trailer. In the old days (dating myself here), they used the gray hard plastic pipe or the tan stuff, both of which broke easily if not winterized. (Don't ask...)
If you actually are camping in the trailer and you get hard frost, you most likely won't have freezing problems in your unit because of temperature radiated out of the trailer. It's the same effect as a frost at home. Stuff right up against the house won't have frost on it because of heat radiation. Only takes a couple of degrees difference to see this.
I generally don't winterize here in western NY State until after mid-October, although if the weatherman tells us of a unusual low temp situtation, I'll do a bit of dry winterizing to protect as everyone has indicated above.