I think that Bill has spot-lighted the unfortunate facts about any heater in a Panel truck (and the truth is truer as the outside temperature gets lower). It is simply harder to heat the inside of a panel truck (or Suburban).
Three years ago, I drove a 1954 1/2 ton pickup in 8 degrees F for about 200 miles from NJ to Albany, NY. The pickup had a recirculating heater, no door seals, and no window/vent seals. With the heater on high, I had to take off my coat, gloves, and hat. I was, as they say. "warm as toast" (60+ mph and windy).
What a difference the size of the cab makes.
