We took our 14ft expanda outback over the Great Central Road (GCR) 1200km of dirt, stones, and corrugations last year. No problems at all, but in saying that i did alot of work on it to dust proof it, including positively pressuring the van, alot of stone protection. Stone guard on front, large mud flaps attached to the stone guard, added mud flaps under the van to protect, shockies, water contections, water tanks. Covered all the pvc drainage pipes with rubber tubing. I also sealed all the wiring that was just tucked up under the van on the drivers side into an electrical sealed box. Had a graat trip, but did take a few washs just to get the RED dirt on the van itself. Make sure you take an extra spare with you, we had two for the van (only single axle) and two for the car, did a tyre the first day on the GCR.
Ps we are looking at doing the GRR next year, so interested to here more from all