Better suspension systems for a start. Independent suspension with decent shockies. Some profess to sell off road, but a few only have shockies added to the standard axle and spring arrangement, and lifted them a bit. Another thing to be very careful about is the build quality of the interiors. Some still appear to have only the same build quality of black top vans and would probably fall apart on long stretches of heavily corrugated roads.

Have a look here to see some different suspension types.
Off road caravan suspension

To be honest, I have only looked at a couple at caravan shows, but the ones that appeared to be better used plywood and timber rather than chipboard and were glued and screwed instead of using staples and glue.

Hello

I would suggest have a look at Kedron and Bushtracker Caravans they are not cheap, but well made and designed for off road use.
To get a reliable off road caravan you need to pay more than you would for a standard tourer and much of this extra cost is in places you do not see, but really matter in tough conditions. I would suggest looking at owner forums and see where the members travel in their caravans, this is often a good way to get a feel of how off road capable caravans cope with tough conditions.

We have a single axel XC3 Compact Kedron on order for delivery in August. It is small enough to be towed by a Prado or Pajero legally. After much looking around we decided to go for Kedron. It is a choice for us of paying extra for a tougher van that we can be confident in taking off road. Both the above vans have good web sites and if you get serious I would strongly suggest arrange a factory tour. It is there you will see where the extra materials and effort (and cost) that goes in making an off road caravan.

Good luck in your search.
Terry

Our Trakmaster had solid timber (meranti I think) framing, and all cupboards etc were glued and screwed to it.

Underneath wiring/piping was either fastened very securely where it couldn't snag, or in the chassis channels.

All fittings were as strong and solid as were available then.

We travelled lots of rough roads, and corrugations, and nothing fell apart, or gave any problems except for supplied components (fridge door fell off; stove glass top parted company from its holding hinge).