It would be good to hear from the OP as to how he fared overnight. I've had RV's in the desert for more than 10 years now. While it rarely gets very cold, usually it gets below freezing at least once a year. I've had one 1/2" line freeze when it got down to 26 degrees. That line was in the outside compartment near where the water inlet is located.
I also had the icemaker polyflow line freeze up once when it got down to 27 degrees. I've found that the outside compartments gain protection if you stuff rags in any vent louvers, plug openings for hoses, etc. I keep indoor/outdoor thermometers in outside compartments and inside the coach so I always know EXACTLY what is going on.
Potential damage is largely dependent on size of water line and how long the temperature stays 5-6 degrees below freezing. You definitely CAN freeze a water line solid enough to rupture it if the temp stays below 27 deg for more than just a couple of hours!