Yes your way off you’ve worked on about 7 times the fuel usage...

Yes your way off you’ve worked on about 7 times the fuel usage of a heavy sedan weighing 2.2 tons.
These trucks will gross out at about 150-160 tons with a 700hp power unit sucking diesel at idle.
A 40 ton payload B Double (60 ton gross) can travel 1.6km/litre
So it will be more efficient with a bigger unit but only by 10-15% which is still a lot over the tons
Fuel will be diesel, price is cheaper than a bowser as it’s bulk supplied excise rebated along with GST but it’s outback delivered so that’s a cost to get fuel there.
Ball park a contractor (and this can vary on contracts) will charge $7 per Km on long haul (this is a road train with two trailers) short hauls are more expensive because of loading and unloading is more frequent. (FFS don’t suggest it should be cheaper) These are expensive units to buy and maintain and equipment is in short supply and the second biggest cost is tyres. It would take half a 44 gal drum to change the engine oil and a few to run the hydraulics.
My numbers won’t be exact but you can work on them.