In below freezing weather, I generally sleep with no heat on, and turn on the RV heater when I wake up. I store a crate of 4 gallon jugs in front of the heater outlet. That is by coincidence, it just happens to be a convenient place to store water, but the heat does help to keep water liquid. A gallon jug or other large jug seems to hold heat longer than a small bottle. Generally, if it is cold, I am moving, driving somewhere else during the day. Then the motor heats the cabin. If you are stationary, that doesn't help you.
If you are not in the van during the work day, everything in the van will freeze solid in severe cold weather. In that situation it is probably best to carry a backpack with water bottles with you into a heated area.
At night, the warm spots in the van are under your quilt and above your head, where your warm breath rises. You might keep a 2 liter bottle by your side under your quilt on a cold night.