I use to average 16.5/100km in my 2001 4.0L v8. I know the claimed consumption is around to 10-11l/100km mark, but I have never been able to average less than 12 in the daily city running. When doing long trips I have managed as much as 1000km per tank.

I have switched off the auxilary heater and see how I go with the consumption. I think I will have the unit unplugged as long as it doesn't throw any error codes.