We are using a Webasto Airtop st3500, its a diesel or kerosene forced air heater, very efficient, doesn’t suck down power like RV furnaces do, and quite compact.
The van is a VW transporter T5 TDI400 Diesel with a 1100kg payload. 4motion which is VW’s answer to all wheel drive provides much better traction on loose road surfaces. The mid roof raises the inside height to about 1.6 m.