When I had a similar problem, (a "Do Not Shift" display on the shift pad), CoachNet paid the tow to Detroit Diesel near the Mall of America, (the nearest "authorized Allison service" dealer).
Four days later they had it fixed.
After trying most everything else, they finally had found one BAD wire.
That wire carried 12vdc, (but not enough amps to allow the shift pad to function).
They replaced both the pos and neg wires from the batteries to the Allison VCM, (vehicle control module), which is located up under my dash.
The original bill was over $2400.00.
I refused to pay that..... for 40ft of 14ga wire, 4 connectors and labor, (NO OTHER PARTS)!
They finally reduced it to a MERE $1700.00!
I am sure they were able to fix the next one in less than 2 -3 hours.......
and I paid for their education!
'96 Safari Sahara