Movement there isn't unusual, the two sections (cabin and tub) are mounted independently on the chassis. It's designed to move a little - but obviously not a huge amount. Any indication of the amount of movement? I'd estimate mine at no more than a couple of mm.
We carry something like 200Kg in there when travelling - 5 jerry cans of diesel, an Engel 60L fridge, Honda Eu20si plus 10L of unleaded and some tools. I also have a long range fuel tank (150L) and our caravan (19' New Age Manta Ray, 2550Kg) has about 130Kg of ball weight. Ours is supported by Old Man Emu Dakar dual-rate leaf springs.
The chassis cracking happens for a number of reasons. The first is overloading with air bags. The chassis is not designed for a single load point between the leaf spring mounts - and the usual culprit is towing something heavy and having its ball weight push down hard when the rear of the car is rising rapidly - for example, when traversing a causeway too rapidly.
The chassis could also crack when using the wrong towing hitch with load levellers. There were two different types of hitch sold up until about 2011 - the first was a Hayman Reese unit badged as 'Nissan' which only had 3 mounting holes on each side. The second was the Hayman Reese unit retaining its badging (and the additional price tag) which had 5 holes on each side. The former had a weaker grip on the rear of the chassis and would cause it to crack - there was a recall on these (click
here for detail). Nissan responded by replacing the hitch with the 5-hole version and having the chassis welded.
We have used a weight distribution hitch in the past but we recently had a weekend off and took the van about 30km away with no WDH and the lighter ball weight was easily managed (we used to tow a 1.8T van - the one currently in my sig - which had a 180Kg ball weight). I am keen to try a longer trip without the WDH and see how it fairs.
