Air-conditioning relies on fans as well as the actual refrigeration system to move the chilled air around the cabin. If the fan isn’t working, that won’t happen. So a check of the fan in question would be a good place to start. Perhaps there’s a wiring fault, maybe the fan is intermittently failing to kick in.

There’s another possibility, too, and it’s more common than you might think. Some air-con systems tend to build up ice inside the system’s plumbing. If that happens, you’ll suddenly get no air-con and possibly even zero airflow through he vents as the ice blocks the trunking completely. The solution is to turn the system off at that point, allow everything to thaw and then turn it back on. Try that process next time the vents fail to see if that fixes it (temporarily).

I wouldn’t go on a trip without fixing the issue. I'm surprised to hear that there is no stock in Australia, but I would reach out to Isuzu Ute Australia head office if you've only tried dealers so far.

UPDATE 12/2/19: Greg advises that this part was replaced under warranty shortly after this response was published. 

We've also heard complaints about the Jeep but the new Pathfinder is impressive and gets The Tick from me.