Just as a contrasting data point concerning "proper" RPMs, my Cumins ISL (370 hp version) runs very comfortably at 1300-1400 RPMs and cruises the interstates at 1500. Peak torque is reached at about 1250 and stays flat til about 1500. Peak horsepower is reached at 1800, so 1500-1700 is a good operating range for hill climbing.
The ISL is a bigger engine (8.9L) than the ISC or Cat C7, so it can run at lower RPMs without lugging. Conversely, it cannot run at as high an RPM as the smaller displacement engines, so is governed to 2200 max vs the 2400 max for the ISC.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC