I want/need a camping trailer to tow behind my Z8 along the line's of an expedition trailer, do I need to add suspension or will the atv tyres be ok to absorb the bumps on/off road ?

The suspension is set a little stiffer in the rear already to handle a load. I think they say they can tow 1500lbs safely. You have a lot of extra stiffness adjustment available in the rear from stock if you need some more stiffness. If you keep it light I wouldn't worry.

If your talking about the Z's suspension then yes you would need to add preload.  If your talking about the trailers suspension then no just put ATV tires on it.  Now if you want to avoid ALL of that and have a trailer that pulls the best off road that you can have and have it put little to no weight on the Z then you want to do a "walking axle" trailer.  I built one a year or 2 ago to haul logs.  It is amazing for anything off road because it self levels and balances the bumps and loads out.  I have put several thousand pounds on behind a Zforce and it pulls it right out no problem.  Here is a video of what I am talking about (not my video but shows how it works) https://www.youtube.com/watch?v=0j6OwkBqsIo

Sorry yes I did mean suspension on the trailer 

Nice trailer setup. I love it. Now you got me thinking about them.

Walking beam trailers are AMAZING.  They cost a bit more and are a bit harder to build but they pull SOOO smooth.  You dont hardly feel them crawl over thing that would completely stop most trailers