The problem with travel trailers is they are built cheap. I have a 40 foot 5th wheel from 1987. Its 3 axles, and 9500 lbs dry 11K loaded with water and food stuff. But the whole thing is built from wood. Well, except the chassis is steel, but without shocks on the axles, it bounces everywhere on corrugated dirt roads. It shakes the crap out of the inside. I've had to reinforce all the cabinets inside.
The difference is I don't have all the fancy TV's and electronic crap and they do. I paid $4500 for mine, and they paid $75K.
Even my in-laws 36' 5th wheel isn't built that well and they paid $75K for it. and its a 2016 model. Weighs 12K empty and is a dual axle.
I like to set my 5th wheel up in a area I want to explore, kind of a base camp, then spend the week in an area exploring and having fun. Having room inside it is important to me. I need space and places to sit. We have 4 kids, and trust me, sometimes you want to be able to close a door and take a nap away from the noise.
I think the best way to build one would be out of a full aluminum chassis and frame. Then skin the outside with aluminum. Built a complete independent suspension system with shocks.
The Aussie caravans are very cool. It seems like they are built very well. But the door is on the wrong side, connects are backwards, etc. But are they worth it after shipping it here? And how about getting parts for it. And Import fees and taxes? It all starts to add up.
I don't think you'll ever find what you want without some compromise.
If you want some help importing them, I know a bit. My friend and I have been importing Land Rover Defenders for a bit.