Iain_U1250
If Australia is one of the place you want to visit, you could probably use your exisiting set-up without the trailer providing you stick to the more touristy places. There are always large buses and trucks going up to Cape York, Mitchell Plateau and Fraser Island. Basically the dirt roads are pretty good, with the majority of locals using 2wd station wagons for transport. Your need to be able to have a 800km range, but 1500km would be much better. The Outback towns run out or fuel or close for the weekend, but generally they are around 350km apart. Camper trailers are very popular, but I have not seen a truck and trailer out in the remote areas yet.
If you want to go to the more remote places, then any much bigger than a Fuso or small Unimog will either not fit, or you run the serious risk of staking tyres as the normal Land Crusier/Patrol wheel track is what sets the limits. Any wider and you are driving outside of the ruts or on the edge with all the sharp sticks. Height is not so much of a problem here, but approach and departure angles are very important for creek crossings and sandy tracks. Tracks like the Canning Stock Route, Gunbarrel Highway, Anne Beadell require a lot of permits and trailers are banned. We had to fold in the mirrors on our Land Rover on the Anne Beadell as the track is quite narrow in places.
Australian quarantine is very strict and nearly everyone ends up having to get extra cleaning done. I would definately not just try and bring in dirty truck, the regulations have a maximum amount of dirt allowable, and if you are over it, they could just put it on the next available ship to the port of departure, at your cost.