I've camped several times in freezing overnight temperatures. Only once have I closed the unit down during colder weather. We ran the propane furnace and the electric heater on high until the very last moment. This helped keep the vinyl pliable. We moved pretty quick once the door was stored though.
Since we use the water system, we drain the pipes for the night. Simply shut off the pump or disconnect the city water line, open the sink faucet and the low-point drains. The water tank wasn't in danger, and the water heater was full and on so no risk of freezing there! If camping where hard freezing is possible, we fully drain the entire water system and use bottled (tap) water.