I did that trip last year John, but East to West. Used Camps 6 to free camp from Darwin to Broome via Catherine for that entire leg good camp sites along the way. I could have used one just south of the Broome/Perth junction too but weakened and stayed at the junction roadhouse caravan park for one night instead.

Used the towns and roadhouses (and at Victoria Crossing the caravan park shower, small fee) for showers etc along the way.