It certainly was the case that automatics were very significantly worse than the same car with a manual gearbox, but this was in the days of 3 speed automatics, kipper ties, and nylon underpants. My father had a petrol 3 speed autobox E230 in the early 80's, and the gear changes were just unbelievably bad, so much so that I never considered that company or an automatic for 20 years. And don't even think about fuel consumption...
But a modern 5 speed automatic gearbox coupled with a large diesel engine, both controlled by a well designed engine management system, seems to be the perfect combination as a tow car.
For instance my 2 year old C270 CDI (185 bhp, 450NM torque) automatic returns around 46 to 52 mpg solo, and between 33 and 38 mpg towing. Looking at the book, the manual version improves on these figures by 1 mpg in the city and 3 mpg extra urban when driven solo.
This is an almost insignificant difference, and the ease at which the car drives - both solo and towing - makes me wonder why I didn't get an automatic sooner.
Robert