I'm not too worried about the pex but I'm scared of the fixtures busting. Here's what I do:
Flush and drain the black tank, drain the grey tanks
Drain the holding tank from the low point, then turn on the water pump and make sure it's pumped itself dry
Take the anode out of the water heater and let it drain, then replace it. (I always shut off the water heater when it's parked even though I keep the power plugged in.)
Hook up the air compressor (regulated to about 40 PSI) to the city water connection - I homemade a connecter with a ball valve on it, I make sure it's closed when I start
I open the closest hot water faucet(bathroom sink) and then open the air valve, then open the cold and shut off the hot, then I work my way through the system to the furthest faucet always having a faucet open while the air pressure is on.
Pour 1/2 gallon or so of RV antifreeze down each drain to fill the traps
Take out old water heater anode and toss it, replace with a new one, with new teflon tape.
I dunno if any or all those steps are necessary but in 4 years I haven't had any trouble so far. Except for one year we went out a little early and had a freeze after our trip, it screwed up the water pump.
Oh and you water filter is NFG after it freezes, so remember to replace it after a freeze, I would use a new one every spring anyway...