Bit of an update....
Clunk still there - on the road, with or without camper. Did a 4WD towing course on the weekend. Did not hear it at all in low range.
Went for a drive with local workshop when we got back and the clunk did its thing. It sounded to both of us like it was around rear diff/suspension.
Mechanic put up on the hoist and used a pry bar to try and find something to move that shouldn’t. Worked all around rear but everything seemed okay. Working forward got to transmission and they could lift it probably 50mm !
Called me later that day, transmission mount had 3 cracks in it. Ordered a new one and they are in stock in Melbourne. 10 DAYS DELIVERY - TO SYDNEY ! Seriously? Hate to think what it would be to Darwin or Perth.
Put it all back together, and also fitted Chief Products transmission protection plate for our trip. Hasn’t made a clunk since so looks like we’re on the right track. Will update after new mount fitted and taken the camper away
Thanks all for the suggestions. The list was handy when talking to the workshop👍👍👍👍👍