Whenever you do heavy towing it is best to add another transmission
cooler... it will extend the life of the transmission and reduce the wear
and tear on engine... have you checked if you have enough tranny fluid?

Pulling a trailer makes the engine consume a lot more fuel and the
radiator needs to shed the unused / wasted BTUs in that extra fuel so
there's no wonder that the coolant temperature is higher than driving
w/o the trailer.

I wouldn't be concerned about it so long as the temp remains below 120
degrees C.

You can clean the radiator's outside with a garden hose and nozzle or
remove it and have the inside passages and the outside coils
professionally cleaned by a radiator shop.

Of course, the electric radiator fans - which you didn't mention -
should run when the coolant temp. exceeds 100 degrees C.
Do they?

Yes,  the car runs hotter when towing because it's working harder and
the Tranny oil most likely does also go through the radiator.  If your
car has an oil cooler it should open and start cooling the oil at about
110C  so as long as your temperature is around there (not over 120C)
the car is working correctly.

Marty

Is it normal that a diesel engine  - even when running in the best torque
interval - produces a relative more heat than a gasoline engine?

I also have a Citroen Xantia 2,0i, (gasoline) automatic (no tranny oil
cooler). This car hass less torque even at a higher RPM than my Mercedes.
The Citroen needs to kick down more often when towing the same
caravan/trailer at 85-90 km/h, but the temperature seldom increases - even
in hot weather 30?C - with more than 5-10?C.

Anyone with a good explanation on the difference in behaviour between the
Cit and the MB??

Jens