Hi Tassiemav,
Some camper trailers have the fridge & whatever in the forward box balanced with a 120lt rear water tank. You could imagine the weight shift onto the ball as the water is used.
Usually it is a good idea to upgrade your suspension when towing a camper trailer etc, however if your ball weight is only 85kg you could use Polyairs when towing & still have a decent ride round town by letting the air out to the minimum pressure.
The Polyairs fit inside your coil itself
http://www.heasmans.com.au/new_site/products/a1000.html
Your spacers fit on the end of the coils.
Rob