My engine, (Cummins 5.9, ISB 260hp) operates in the range of about 1600 to 2600 rpm. The computer and the transmission (Allison 5 spd) keep the engine within those limits.
When you engage the Exhaust brake at hwy speed, the transmission is told to go to 2nd gear, but it will only downshift when the engine will not overspeed. It goes to 4th gear and holds that until slow enough to go to third without allowing the engine above 2600. As the operator you can't change that protection.
In your case, 1st gear, as rpm increased, light load, the computer would shift early into 2nd and drop the rpm toward 1600. with a heavy load, (steep hill) the upshift would be later closer to 2600rpm, as Driver discussed..
Hope this helps...
Hooligan, Pensacola, Fl -U.S. Coast Guard Retired
2016 Thor Siesta Sprinter 24ST diesel
Our Pug "Lily" & "George" the Newfoundland