Welcome to the forum!
+1 on overdrive. Use it as much as possible - if you have an auto, try to keep your speed above 75km/h on the highway, so that the torque converter locks and engages the engine-gearbox directly. The lower your RPM, the less fuel you're using. The auto box manages load vs speed rather nicely so just whack it in D and forget it.
We tow a slightly lighter van (500kg less) than yours but our Navara is a little heavier - about 2500kg without a load - so we're probably close to even on weight. We get as low as 15.5LPHK when we're gentle, 17s-18s for a moderate cruise and up to 24s when the going gets tough. I've yet to break 25LPHK but I haven't towed the van across vast tracts of sand - at the most we found just a few soft spots on the Ernest Giles Road requiring an excited heavy stomping on the accelerator. I don't expect the Birdsville trip via Windorah to be a lot different, although we'll be leaving the van @ Birdsville for the trek out to Poeppel's Corner (if we decide to go there).