Any truck shop (Freightliner, Spartan, independent, etc) or Cummins or Cat service facility can do routine service and most likely any major repair as well. Speedco service centers will handle motorhomes as well as trucks for oil changes.
For a motorhome, plan on annual oil changes, or annual oil analysis to show that a change isn't needed yet. You won't drive enough miles to worry about miles per oil change. It's probably upwards of 10k for any of those engines, and more likely 15k.
Even the sleeveless engines are good for at least 350k between rebuilds, and motorhome use is considered light duty anyway. I don't think you need worry much about that.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL