I would not tow a trailer of any description through the Fraser Island tracks in an x-trail, up the beach on the hard sand maybe but as soon as you get off the beach into the tracks you will struggle for sure. We had an x-trail which we owned for only a year and for this very reason we sold it and bought a Toyota instead. I didn't want the worry of the x-trail getting stuck or having expensive gearbox/clutch repairs as a result of towing trailers through soft sand. I researched the web for months and the stories of burnt out clutches, gearboxes overheating kicking it back into 2WD and so on were endless. They have no low range, insufficient clearance for the soft inland tracks and are not designed for towing heavy trailers through soft, sandy, bumpy island tracks and you will get stuck. I'm trying not to sound like a pessimistic old woman here but the inland tracks at Fraser are 'soft and rough' and in some sections quite difficult to navigate in a dedicated 4WD.
In particular you could have major problems navigating the Indian Head bypass and the other beach sections north i.e. Waddy Point, Orchid Beach etc.
If you are going to go then make sure you travel the beach at low tide so you can stick to the hard stuff and camp on the beach, don't try the inland tracks and whatever you do go onto the island via inskip point, do not even attempt Kingfisher as you won't make it up the first hill.
Cheers, PJ