Another easy electrical check:
Most coaches have a "pre-select" gear when exhaust brake or engine compression brake are activated (not just in "stand by). So, your Allison shift pad will go from "6" to some lower gear-0 with 2 and 4 being common choices.
So, when you are driving, what does the left/only Allison shift pad indicate?
BTW, engine compression brakes are VERY common on the ISL in RV applications. Have exactly that engine in our 2003 Alpine (Cummins ISL, 400 HP, 1,200 lb-ft torque at 1,300 RPM and two stage engine compression brake). A very good power to weight ratio in 38' two slide coach.