Using a Big Buddy heater with 20 lb tank. Works great avg around 95 hrs per tank on low. Keep window open a couple inches and use a carbon monoxide detector. Only pain is moisture on inside of windows.
In this weather your Sprinter just won't produce heat at idle or low speed. Its not the van. You can try an idle stick to bump it up but that's not economical. Can't help with the rest, good luck.