FFS i think you may have a problem...i got more than that towing a box trailer on me L series...ok i admit its a manual, and i couldnt go fast but i think my average was almost 30 mpg on the trip...maybe you should find a diesel specialist and get it tuned...when i used to run petrol cars, part of my own annual service was to get a Crypton tune maybe it needs a similar boost, especially if youve had a gadget to alter the fueling put on..
D