It would take a lot more than a few degrees below freezing overnight to freeze a p trap solid enough to do any damage.
A coach sitting in AZ would have a reasonably warm interior during the day, even in the winter. To freeze the p trap it would then have to first lose all that residual heat.
I've used a recording thermometer in my unit here in Canada. On a bright sunny day where in outside temp is ~32F the interior is ~45F. If the temp drops to ~25F overnight the interior will not drop below ~30F and that must be for a very brief point just before sunrise.