I wonder in these posts when I read "water freezes at 32" why the poster has to state the obvious.
The real question that needs answering is "how long does it take the water to approach freezing?"
I live in SoCal and we have occasional evenings where the temp might approach 27 or 28 degrees. However on those evenings it's been 60 + during the day in most cases. Also, it's going to go back above freezing as soon as the sun rises.
Even though the evening temps will be well below freezing for a short period, it will take a significant amount of time at that temperature for water in any volume to actually freeze and the chances of any damage in an RV water system is non existent.