Its worth trying a run of say 150-200 miles, on a motorway, keeping the speed to 65 MPH max, ideally 60, just faster than trucks.
Driving carefully, ideally using cruise control, this should get you around 25-28 based on yours being an auto. At least you will then get a baseline figure.
Above 65 the (lack of!) aerodynamics start to come into effect, above 70 and they have a bigger influence still.
I do most of my driving at 65, my car is clean, no roof racks, lights and its lighter, no back seats or bolt ons.
I average 30-32 MPG
If I drive nearer to 70 its about 28-30
As pointed out before, if you brim the tank the gauge wont move for about 100+ miles, so make sure you fill it completely, and ignore the fuel gauge!
Bottom line is, its an auto, its got added weight, bolt ons etc.
You also pointed out that it seemed to be revving higher. have you done a stall test on the auto?
Mine is manual, with 18" 255/60 which are 1" above standard diameter. at 65MPH it revs are 2500 RPM.
Driving technique can make a huge difference, more so on auto,s, my old boss always drove with a "pump/brake" style, which infuriated me as his car never got more than 25 MPG, when I drove it I got 36.
Mark

I can only add what i get with my disco ( standard trans, TD5 372,000km (231,000 miles ) on the clock,oversized tires 265/70R16) nothing hanging on the body. I get on average 30miles per gallon over years of monitoring. Even with the rear brake caliper stuck (did not notice caliper sticking until smoke came from wheel) did not change mileage or performance. even with a trailer get 27 MPG.

I would personally be inclined to check the operation of your MAF. Most modern diesels will happily trundle along with a faulty MAF, but as fuelling will be set to a pre-determined last known safe setting, mpg will suffer. Most diesel owners don't even know they have a failed maf, because they rarely show a fault light or code until they fail all together.

They have a massive effect on mpg.