Given the CEL I'd suggest Nissan can pay attention to it. Ordinarily they'd tell you to suck it up where poor economy is concerned, but with the CEL present they'll fix that and hopefully whatever is bringing the CEL on is what's causing your poor economy.
If not - and the CEL is gone, service done and you're still getting poor economy, we'll start talking about what might be.
I know the V6 TD is an awesome engine, develops its torque nice and low (the lower the peak torque, the better the economy on the highway). However, reports from others towing on the heavy side (on flat-ish terrain, admittedly) are that they're getting 12-13LPHK. 16s are a little high, unless you're talking mountainous regions, lots of small hills with winding corners or the Great Ocean Road which I'm only ever going to do one more time in my life since it's little more than a bloody car park during summer!