Hi there,
I have the 1KZ-TE with full time 4WD and automatic transmission in our 1998 KZH138. We are hauling around an old school camper build out from Japan and weigh in around 2,600kgs with our gear and kids inside.
We have calculated an average 12.5l/100kms combined town and highway, so it sounds like you are around that range. I had a 'modern' 2009 Land Rover for a few years back when it was new and the consumption was way higher than my old hiace camper.