We are currently on the road, but took our 21-64 OB on the Gibb in July. No prep as decided very shortly before going that we'd take it. (Can't take it to Mitchell plateau). Came out fine, just recommend no heavy saucepans in the pot draw under the griller due to the way jayco install the draw runners. (In my opinion, the Gibb was fine, a bit bumpy and rocky in bits but you get that- lowered our tyres to 28 all round while hot, turned out to be between 23 and 25 cold with no problems at all). Also drove it thru NT and thru cape trib. A bit squishy, but still ok.
Picked our van up dec 2014 so is still newish.