We camped for days with temps in the teens many times with uncovered/unheated tanks hanging below in our old Sprinter. I used 1/2" pink styrofoam and foam gap filler to insulate the fresh water tank, applied about 3 inchs of foam to the exposed pipes and painted them black. Always put a cup of RV antifreeze down into the black and grey tanks (not just the traps), used a 60W bulb over the water pump in the unheated basement, and used the fresh water tank instead of the city water tap, filling the tank in the daytime and then draining the hose. Worked for years!
part-time workampers since 2005; about 6 months/year. Tow vehicle: 2001 Dodge 2500 diesel. Recently upgraded to a 2008 Cruiser 29CK from a 2002 Keystone Sprinter.