We have a van that is left to the elements over night.
We switch off the tap from the tank, unscrew the filter bowl and empty, and switch the pump on. The pump will empty itself by sucking air and pushing it through the pipe to the van ports. We use a hose joiner to let the water empty at those ports onto the ground.
We bring the hose reel inside. The pole hose is also drained so there is less water in there to freeze if you can't bring them inside. I would cover the tank with a duvet as has been suggested.
You may have to run the van and warm the back up a bit to get you going in the morning. A hot towel wrapped around any frozen pipes does get them thawed reasonably quickly.