We managed to survive without any damage to our water system on a night (in Bisbee, AZ) when the temperature dropped to 4.5 degrees. I check the weather on a regular basis and knew the cold weather was approaching. In anticipation I disconnected from city water, opened the gray tank drain valve along with several other steps. The only issue we had was with the bathtub drain trap freezing and the bathtub water taps freezing. Given the plumbing is a Pex system no water lines were damaged.
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To answer your question - it depends on your preparation. I think temperatures in the low 20's would not be an issue.
Dick