Camperpaul's solution is the best idea. We fulltimed in our home locations for a few winters with temps -20...the absorption refrigerators can quit at that temp so steps do need to be taken to correct that situation. Our's shut down and did not restart until the weather warmed enough for the unit to thaw. After we got it going we followed the same steps as camperpaul and never had a shutdown thereafter.
If you shut off the unit then how will you cool food? We've used coolers in the back of the pick-up...not ideal for sure.