I am not sure what you are trying to accomplish or save. Do you want to run your van engine and use the dash heater to heat the van. That sure that works up front, but the bed is generally in the back and if the night is cold, you will need to run the engine through the night and spend a lot more on gasoline than a generator will cost to run. You could get a generator and run it outside and connect your power line to it and run an electric heater that will nicely warm the inside of your Class B - if it does not have its own built in generator. My Class B has a propane furnace which has cost me little to operate. In two years I have put about ten dollars in propane to keep the tank refilled to full - and that has run the furnace and the hot water heater. My Class B also has a heat pump which will run off the generator or shore power. The propane furnace is vented to the outside of the van. This is extremely important for your safety inside for any heater that is not electric.
Sometimes to save a few bucks one loses much more - life.
