First, the dipstick is only useful if the temperature of the fluid is known.

The fluid change is not neccesary per MB, I would go along with the MB recommendation. I have seen these transmissions last close to 300,000 without a fluid change. This particular example was in a E300D though, not an ML, (as far as I know it's still working good in that car), so it is possible to get the kind of service life without the change, at least in a car, no towing.
MB doesn't state anything about changing the fluid based on the fact you tow trailers or any other severe service.

BUT it IS your truck, so change away if you want, it's not like MB recommends "against" changing it, they just don't have a service interval for it. Lots of guys change differential fluid for example without there being a service interval, same thing here I guess. Rotate the air in the tires too if you want.

If you do happen to do it make sure you get the rather pricey MB fluid to do it with, the filter and gasket are relatively cheap, and also there is a magnet that MB wants placed in the bottom of the pan if the opportunity arises, use the search function to find the part number, I just posted it last week.

Gilly