Actually the Aux heater that is installed is an Espar (or a correct name is Eberspächer) heater. I think it is a model D5W in the Sprinter and is very well proven outside of the US. It is a water heater. Eberspächer makes a lot of heaters, both for water and air and both gas and diesel.
If you want to really get some heat, you can get an Eberspächer heater for a Unimog or a gas winter heater for a MB38A1 jeep. I had both and the Unimog heater did really well to about 10 degrees but the MB38A1 heater always put out too much heat and always cracked the windshield. It even heated my friend’s cabin for a few years, no matter what the temps were, it was 70 plus in the cabin. Used a lot of fuels though.