+1 for Chris' answer.
I believe for wof requirements it's the laden (gvw) weight.
Checkout out www.rvwholsalers.com
as they generally list both the dry and gvr for each model they sell. I'm not saying you should buy from them, but it's handy info. They are cheap though!
is a USA forum very similar to this one and there are tens of thousands of posts under 5th wheels there, and they are very quick to answer any questions you might have.
Wiring to NZ standards is an issue and a cost that has to be allowed for. In saying that, it's not that big of a job to rewire, but I know there are some sparkys out there that see this flash new 5th wheel and charge more/take longer because they believe the owner will pay. Sad but true. You can legally use a transformer as long as it can be fitted within the actual appliance it is powering. For example, the fridge that your new 5th wheel comes with - you can fit a tranformer behind the fridge and as long as it's enclosed within the fridge (which is easy) this is legal for an electrical wof. Other appliances like microwave just aren't worth worrying about, just replace with NZ ones. Now this is the ruling as I believe it to be but I suggest you get your own advice on it