I've only done one touring type trip, so my experience is limited, but I'd still definitely take:
at least 2 spare wheels
I had 50L of water and
provision to carry an extra 80L of diesel (only took 40, didn't end up needing it, the stock 75L tank got me everywhere)
All the spares/tools listed above.
60L fridge
Long life milk, long life juice (doesn't need to take up alot of fridge room then)
Camp cooking gear (I've got all this packed neatly into 2 boxes, so it's grab and go) this entails a 2kg gas cylinder, Coleman 2 burner cooker, billy, small saucepan, big cast iron frypan, BBQ plate for the cooker, one of those cheap butane cookers plus a heap of spare fuel cells. Spray olive oil. Thats in one box, in the other I have cutlery, bowls, plates, mugs (all stainless steel - lasts longer, easier to clean) disposable plates and cups, milo, tea, coffee, sugar, salt, pepper, paper towel, chux, dishwashing liquid, etc (you get the idea)
Camp chair
Spend $20 and get one of those fold out toilet chairs (oh so very handy!)
The usual recover gear (take spares of a few things like shackles and at least an extra snatch strap, make them easily accessible when you pack them in)
Long handle shovel
Hi-lift jack
Some cheap tarps (makes excellent drop sheets if you need to work on the car)
Easily accessible towel, spare change of clothes and first aid kit (behind the back seat is good for this)
Swag/tent/stretcher
Spare batteries for torches, etc!
Toilet roll (again, make it easily accessible!)
Camp table
You'll find your own list of things to add or take from that, my only advice is to spend a lot of time thinking about what you take, and how to pack it in the nav. Believe me, it time very well spent.
And above all, let us know!