It's called DPF and regen. That RPM is better then the older ones as that truck has a better transmission. To bad they didn't have the newer 6 speeds with the older, pre-emission 5.9's. That would have been killer.
Anyway, by keeping the EGT's low you improve economy, but then you get an active regen cycle more often which dumps raw fuel to burn off the DPF. Driving it a little harder may keep the DPF clean without regen. What is best for economy I have no idea. Delete the DPF and deactivate regen would definitely help.
I see a youtube video that claims 1112 degrees is where passive regen occurs. It also says if the active regen cycle is started and you turn off the motor before it finishes, it will start all over on the next startup. It is probably not 100% accurate for your application but an interesting video.