Our van is 1080kg, towing at 70mph (or above) gets 18mpg
Towing at 55mph gets 26mpg - but usually ends in an arguament.
I normally expect to get 20mpg towing, can't see where the problem is, 18-23mpg sounds about right.

Tried the command shift when towing, tried leaving her in sport mode, given up and left her in normal, works fine, changes up at 2200rpm, changes down at 1400rpm. Tangiers Orange - gone, missing her
Replaced by Ewok what a mistake - now a happy Disco Sport owner

Would think that's about right for a 2008 auto, a later SD4 auto would do better on that weight van. I get about 25mpg with a 1350Kg van @ 60mph, but remember it's not the weight but frontal area as the speed increases my PC is slightly to the right of Genghis
2012 HSE SD4 In Orkney Grey now gone, best car ever.

I have an FL2 58 plate Auto with 55K on the clock. I tow a new Elddis Affinity 540 which is about 1300K empty and I get 24MPG fully loaded for the summer holidays and 4 x bikes on the roof.

Usual weekend loaded towing and no bikes I get about 25-26MPG

But it may go up as my rear brakes are binding a little, which will be sorted this week.

More than happy with that.