Last year we did Perth to Darwin in a Drive Beyond rental. It was a diesel Nissan Navara (Frontier)with a roof top tent. Over the Gibb River road. Even camping and driving off-road in Northern Territory.
Most of US is pretty tame, even out west, with smooth freeways, they simply don't need a tough trailer with independent suspension. Not all "off-road" trailers in Oz are,either. Aussies don't have heavy tow vehicles, see that SUV. So they are limited to 20' class. I was always amazed at the size of the caravans they tow.