But my car is not an 'Eco' car in the normal sense of the term. 'Eco' generally means that a conventional engine and drive train has been tuned for low fuel consumption and other measures included, such as fitment of tyres with low rolling resistance and sometimes aerodynamic modifications, too.
My car is a hybrid which is a totally different kettle of fish. It runs electrically when it's not called upon to provide lots of power and the petrol engine cuts in as necessary when more power is required. When towing, the power of the electric motors is normally insufficient and the car runs almost exclusively on petrol (sometimes aided by the electric motors, as well). The benefit of a hybrid is therefore only realised in the solo condition and more so in town traffic than at speed on the motorway.
The difference in fuel consumption between solo and towing is therefore enormous. Whereas 35mpg or better is easily achievable in the solo condition (even up to 50mpg in a city crawl), which isn't bad for a petrol engined car of this size and a kerbweight of over 2000kg, the average fuel consumption when towing can be as low as 20mpg.
On the other hand, the 211hp petrol engine and/or another 62hp from the electric motors is perfectly ample to pull my 1800kg caravan.