Air bags can be a great help and they can break your Navara's backbone if mistreated.
That's the key - mistreatment.
Air bags are different to leaf springs in one important way. Leaf springs have two contact points on the chassis, some considerable distance apart. The air bag has a single point.
This means that if you mistreat the suspension, all the weight is going to be concentrated in a relatively small area. The load on the towball acts upon the point of contact like a lever, trying to bend the chassis at that point.
If you limit your ball weight to 300kg (max ball weight for the Navara) and limit your Navara's unhitched weight (including passengers, fuel and tub load) to 2680kg then you will NOT mistreat the chassis.
The answer then is a conditional "air bags are okay". The condition is that you do not overload the vehicle - and more particularly, the weight on the tow ball.
I would look at Firestone Air Bellows. They use a heavy plate on the top of the bag rather than a small dome on the top of the bag - that helps spread the load a bit more. ARB can fit these for you.
