Jonathon
You will be fine in your Challenger. You will not have a problem with anything freezing until you are in the teens. Your heat pumps on the roof will not perform well when you get into the low thirties. Your heat pumps will not heat your compartments. If you use your gas heat they are most likely designed to drive heat into your compartments. If it does get down into the teens disconnect your water hose at night. If you keep your water tank full you will have water for showers, cooking, toilets, etc in the morning.