Hi Claire,
You don't say wether you were towing a van, although as this is a caravan forum one assumes you were and you are not specific about consumption figures.
No knowledge about chipping, I'm afraid but if it helps, my Shogun 3.5 GDI SWB does about 20 MPG towing a 1.5ton van. The diesel should be slightly better say about 21 - 22MPG for the same weight of van I would have thought.
If it's more than that you may have problems with dragging brakes - van or car - a fuel leak or an engine problem. Or of course you may just be giving it too much wellie, I have found that fuel consumption rises alarmingly, in all vehicles, not just Shoguns, if you tow at "continental" speed.
55MPH or truck speed seems to be the most economical and It's amazing how relaxing and ground covering this speed can be - particularly in cruise control.
Big car, big Van, big fuel bill - still cheaper and more fun than a Hotel though!
Finally you will always get somebody telling you they get over 30 MPG from a Disco towing a two ton Van or a Chipped Galaxy towing a Swift conker with 7 kids and a boat on the roof at 80 MPH and 38MPG but they are mostly in Fantasy Land