I experienced the same phenomenon myself this morning! the funny thing was tho, I didn't have any heater in!
helped I'm sure bt lack of frost I'm sure....
last winter I had fan heater in there, pretty high at times too. kept van nice and unfrozen, prob cost me a bit tho. felt the fan heater circulated the air better, had all my filters in there too which wanted to be certain would not freeze. this year no filters in van so will buy oil filled rad I think as cheaper to run. also want to insulate and ply van out to keep heat in.
how do u know how much cost daz?