PEX is forgiving and a minor freeze won't destroy it.
I do want to protect the systems though, and so I drain all the water out of the lines. Then I blow air through to be sure they are drained.
The water heater can be susceptible to serious damage from freezing. I have had to replace this little unit on every one of the 4 Airstreams I have owned. This due to lack of winterizing. The tank cracks and is expensive to replace. Actually I found it cheaper to buy a whole new heater.
If you open up the drain plug on the outside, and use a shop vac to suck out the water remaining after it stops running out on its own you will be safe.
You can't put RV Anti Freeze solution in the water heater, as it is corrosive to the tank parts. You can add a bit of alcohol to it if necessary to protect any small amounts of water left in it, but the shop vac method is going to get most every drop.
I like to lube all the hinges and locks before winter as things tend to get stuck when not used regularly. I spray non -silicone lube like PB Blaster or old fashioned oil to get this done.
I also air up the tires, and be sure that everything inside is clean and ready for some storage time.
We try to use the trailer at least once a month, if only to hook it up and take it for a drive as well.
If the weather is bearable we will camp in it a weekend here and there. But to you Northerners, who suffer through the long dark winter, this may not be practical. Sorry, but that is why I live in Georgia.
We have a winter but it is relatively short and considerably warmer most years.
