I've had three Cummins, one Cat, and now a Mercedes, and my vote is for the Mercedes. Service is not a problem, as already stated, any Freightliner facility or Detroit Diesel can service them. There are thousands of school buses with Mercedes engines that are being serviced every day throughout the US.
As far as torque is concerned, there is a mountain on I-77 at the NC/VA border that I measure my rigs climbing ability and my present Mercedes tops that mountain better than any of my previous motorhomes and all were close to the same size engines! I have also been over the large mountains in the Rockies and was satisfied with my engines performance. May be different with others, but that is my experience.