Hi Leslie welcome, I didn't have the Dmax but did have a 2010 Holden Colorado so practically the same !! Anyway towing our 615 coromal full height van of about the same weight you are asking about, we averaged 16 ltrs every 100km. The only exception was coming from Tennant Creek to Camoweal into a head wind and we jumped up to 25 to the 100. I also had the car DP Chiped which I think helped.
Enjoy your travels and hope this helps
PS a piccy of our set up I have since upgraded both the car and van, just waiting on the new van.