First of all, if DC was going to void the warranty for do-it-yourself oil changers, they wouldn't show you in the manual how to change the oil.
Secondly, engine failures related to parts lubricated by engine oil are a rarity. When they do happen, Cummins steps in and I'd be willing to bet they will take an oil sample as one of the first things they do before diagnosing failure. They'll know if it's been in there for 50K miles or if it was ran low and then topped off before brought in for repair. They aren't idiots.
I simply would never let any outfit change oil in my trucks/cars because you always wonder if they overtorqued the drain plug, didn't tighten it at all, pre-filled the filter and dropped in a piece of the foil cap from the oil jug, forgot to put on the fill cap, etc.
I'd spend more time checking over things when I got back home than I would changing it myself and knowing all of those things were done right.