Going by the fuel gauge is simply a guessing game.

Fill it up to the first click.

Do that trip again.

Fill it up again to the first click at the same station.

Compute.

If this is the fully loaded VX with the 240D and an automatic, don't expect fuel economy to be great. Still, I suggest you check the transmission to see if it's slipping, and look to see if your exhaust smokes a lot or smells of diesel.

Also check that your oil level is not rising... which is a sure sign of fuel contamination, which means you need your pump recalibrated and/or your nozzles cleaned, because there's too much fuel going into the engine.