Jim,
What does the tachometer do when the PAC brake is used? The most common cause, for the check engine light to come on, is that the transmission automatically downshifts to fourth gear when the PAC brake is activated and over-revs the engine. We would all like to think the computers would prevent this from happening, but it is a very common problem on many brands of motorhomes and engines (both CAT and Cummins). If this is the problem, an authorized Allison shop can reprogram the transmission computer to not shift down when the PAC brake is activated and leave the shifting to you.
In regards to the oil pressure, I would take it to a certified Cummins service center. Most Cummins engines that I've had run much lower oil pressure - in the range of 40 - 50 psi. I have never seen a diesel run as high as you are seeing and my guess is that you have a faulty oil pressure sending unit, but that is truly a guess.
I hope you figure it out and look forward to learning what the solutions turn out to be!