There is a couple of reasons, the uniqueness of a US caravan in Australia and and the overall cost. By the time you add the cost of the unit, the freight costs the Aussie government taxes, the modifications and compliancing the cost still is at least $15,000 cheaper than a comparable caravan in Australia. Strength and quality I would think would be similar.
I don't think the mods are that great. The door and steps should be pretty straightforward. The door will replace a window, so the hole is half there already. The gas lines will be done by a gas fitter, I am an electrician so the 240v and 12v electrics should be OK. I have a son who is a boiler maker, so the welding for the coupling(hitch) is taken care off.