Diesel or petrol? Neither of you have garages so it's hard to look up without asking ya.
Either way, it's a little engine being asked to push a lot of weight. It does it admirably, but will guzzle fuel to do it.
There are some things that will make it worse. Dusty conditions can rapidly block air filters and without a LOT of air, the diesel will run very rich. The Navara is supposed to adjust somewhat for that because it uses a MAF (Mass Air Flow) sensor to determine how much air is coming into the engine. It also uses a Heated Oxygen Sensor in the exhaust to see if there's any available oxygen after combustion is complete (too much oxygen = too lean).
You could have a MAF sensor issue, you could have an EGR valve issue, there might be something in the injector/pump train.
You might simply be using fuel that's of lesser quality.
I don't put my foot all the way down, don't really have a need to - I let mine rev out to 2500rpm at the most, usually. More than enough for it to reach freeway speeds.