Is the lorry the same spec as all the others it’s being compared with? Does it have the same gearing etc, if it doesn’t then it cannot be compared with the rest of the fleet 
I would get them to do a download of the ECU, this will show the range the engine has been operating in while you’ve been behind the wheel, as long as you’ve been driving it properly it will put you in the clear and then they can look further to discover the reason behind the poor fuel returns.
First thing I would check is the tyres (I know, here I go again
) it won’t cost a penny to look at them, if you have the same tyres as the rest of the fleet, then it can be discounted, but if you have different tyres the rolling resistance could be the culprit.
Now you’ve got to start looking at the health of the engine, first step here is to check the intercooler for leaks, it should ideally be pressurised to 35psi and the loss of pressure measured, anything over 2psi in 15seconds and the intercooler is junk.
Next would be to get the valves set up right, this is commonly overlooked, but a regular adjustment will really make a difference to an engine, they should be done every 100,000miles at least and at any time when fuel consumption changes.
If that is good then take an oil sample, you’ll be looking for fuel dilution or excessive soot, both will indicate bad injectors. Soot could also mean that there’s an exhaust restriction or that the intake side has an issue, you’ve already checked the intercooler, but there could be a crack in the intake manifold or something as simple as a rag misplaced in the air filter housing reducing air flow into the engine.
It’s six years old now, so there could be deterioration in the fuel lines, as strange as it seems a fuel restriction will make an engine burn more diesel, so swap all the rubber fuel lines if it has any, they may look good from the outside, but internally they will swell, a bit like blocked arteries in your body.
Fuel economy is not smoke and mirrors, it’s just basic physics, you go through a process of elimination and you’ll find the problem, start with the cheapest things first and work your way through the list, most of the things I’ve mentioned are good practice anyway 