Anything that freezing will cause to expand and break:
Water and holding tanks
Water lines
Washer & Dryer
Ice maker in refer
Black tank flush line
Sink and shower traps
Water filters
Water hoses - best to drain, but they're usually OK
Pull the water drain valves on air tanks
Water pump
Take out all canned foods
Unhook your negative battery cables unless you have a true battery disconnect switch
Put pink anitfreeze in the toilet so the toilet seal doesn't dry out
Leave the shower hose/head open and hanging down so it drains
The outside shower
The hot water heater
And no, don't put a humidifier in it - it will freeze and break.
Bill