The solution has to come from Nissan, so that's where you need to direct your concerns. I would seek a meeting with the dealer and express your concerns directly to the service manager, and request that a Nissan factory rep drive your car. Go along on the ride as well. The fact that your car is using more fuel than it should suggests that it is down on power.

From what you say it would appear the engine output is below what it should be, and the high fuel consumption would seem to reflect that. The output should be 118 kW at 3200 rpm and 380 Nm at 2000 rpm. Persist with the dealer, and Nissan, the company has a responsibility to fix your car. Your idea of having a Nissan rep ride with you as you demonstrate the problem is a good one.

I don't believe there are air-suspension seats available for the Patrol. Rather than alternative seats you should be looking at the suspension to soften the ride. Check out aftermarket springs and shock absorbers, and in the meantime put some weight in the tray. In the old days the lads who drove utes would throw a couple of bags of cement in the back.