These are the first diesel engines that I have owned so I really don't know much about them but I would think the Cummins RPM would be about the same as the 6.2 though.
I was just thinking what is the max governed RPM of the Cummins? The 6.2 was around 3000 I believe, if the axle ratio is to low the truck may not have much speed to run the hi way.
My uncle made two vacation trips out to California towing a fifthwheel camper and he said he had no problems running 75 on the expressways.
I don't know what rear gears the Dodge had but my truck has 373 gears so I don't think I'll have any problem with highway speeds.