Hi,
I've written about a similar trip before. Did Murray Bridge to Roowoomba and return twice in 2008 with two adults and two children (12 and 15) and loaded to the hilt with luggage.
The first trip saw us return 4.9 lph which included driving around a when in Qld over the course of a week. We sat on 90-100kph for most of the trip and only exceeded the speed limit to pass trucks.
The second trip was a carbon copy and we got 5.0 lph. I would say we pushed it a bit more on the way home.
The sad part was that both trips were to attend the funerals of my wife's parents, about 3 months apart.
In each case, we filled up in Murray Bridge before leaving and in Dubbo we filled up again. Then on the return we filled up before leaving Toowoomba. The first time, due to convenience, we filled up in Dubbo but the second time we filled up in West Wyalong.
Best of luck with the trip. By the way, we did it without cruise control as ours is a Jan 2008 model SLX CRDi.
Bob
PS My wife, bless her heart, is the one with the heavy foot.