If you are in the camper with the furnace on, you are probably good down to the mid 20's (varies a little depending on where the heating ducts run and where your water system runs)
As long as you are going to re-winterize after the trip, no problem using the water system during the trip.
If you put it back in storage after the trip with water in it...it doesn't take much below freezing before the pipes freeze. You might sneak by if it touches 32F early in the morning for a few minutes but by the time you hit 28-30F for a couple hours, expect it freeze.