You need to be much more specific. Exactly how much fuel are you putting in (in litres) and what is the mileage you are covering between filling up to the top each time?
I had the same engine in my Huge Merc Sprinter and it would regularly return 22mpg even shifting the brick shaped box and a full load around (3.5T GVW so anything up to 4T at times). It could also be as low as 15mpg on a flat out mercy dash across the country or upto 30mpg if I drove like a saint and never went faster than 48mph all day.
Your fuel consumption will depend almost entirely on the way YOU drive. Once you cover the basics like correct tyre pressures, clean air filter and oil changes then there is not much else you can do.
I know I keep trotting out this pic but just so you get an idea of how impressive 30mpg is take a look at this
