Rockhampton to Darwin

My preferences would be: 1. Rubyvale or The Willows - sapphire fields. Buy some buckets of gravel and see if you can find some, or be tempted to buy some jewellery. 2. Longreach - visit the Stockman's Hall of Fame. 3. Winton - check out the boulder opal scene and Waltzing Matilda centre. 4. Mt Isa - Riversleigh Centre; mine tour. 5. Banka Banka station - long day this one. It could be broken into two by free camping at Avon Downs roadside area. Or staying at Barkly RH - costly. 6. Daly Waters - iconic hotel. 7. Katherine - visit the Gorge etc. 8. Adelaide River - war graves and relics. 9. sidetrack to Litchfield NP - see falls, scenery etc, stay at Litchfield Safari Camp CP just beyond Wangi; does mean backtracking to the highway, but well worth it. 10. Darwin - presume you are aware of the many attractions there.
Return via Kakadu - several places to stay to choose from - to Pine Creek with overnight at Edith Falls. Then o/n Mataranka for the springs. After that, presume you will be backtracking to Rockhampton?
We have never had an issue with bad diesel through those parts. It can be rather costly, though, especially at the remote roadhouses. We top up at Camooweal, then make it through to Three Ways or Tennant Creek from there.