http://www.westyventures.com/propex.html
The Propex heaters, a modern RV standard propane heater used in Europe are now available in the USA. There are two BTU sizes available 6500 and 9700. These units have the advantage of a fresh air intake and also exhausting all the waste air. But they do recirculate inside air back into the furnace as well to reheat it saving fuel cost. They have their own thermostat. There are several mounting options including mounting it underneath the vehicle instead of inside. It does have a duct tube so if you wanted to get some heat up to an overcab you might be able to make that happen.
It is designed with a quiet fan.
http://www.westyventures.com/HS2211 flyer.pdf