With my trailer I probably would not winterize and would use the water. Because I know if it gets cold the propane furnace will keep it from freezing. Does the van have a heater that will run overnight and keep the pipes from freezing when it is parked if you do hit a cold spell?
Forecast for today (12/25/15) is 60/35 degrees (day/night).
We don't winterize or worry about pipes freezing until overnight lows drop into the teens, and daytime temps are consistently in the low-20's (which does not occur in Phoenix).
I would not worry about cold weather unless your route includes an overnight stay in an area with the subfreezing conditions described above.
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