Back in those days the Allison Transmission controller (WTEC-II) needed to be isolated from the chassis batteries therefore most all of them were wired/powered by the house batteries. (There is no built-in logic in the WTEC-IIs to prevent starting the engine if the Transmission controller doesn't see power).
Are the house batteries connected and good?
If not, this is why you're not seeing any display on the shiftpad - however, that will not prevent you from starting the engine - IF the chassis batteries are good and grounded well.
You mention power to the dash is ok... when you turn the key does the dash go dim? Any audible "clicks" from anywhere under/at the rear of the motorhome?
As many have stressed in the past, and I'll stress again, grounds, grounds, grounds, (did I mention grounds?) account for probably 3/4s of the problems in older coaches. In many cases you must find where the grounds attach to the chassis, remove them, grind to get good clean metal and re-attach them. Oxidization occurs readily under ground mounting points .
94-Newmar Kountry Star 40-KSDP
Spartan/Cummins 8.3C-300HP/Allison 3060 WTEC-II/25yr RV Tech RVIA Certified/Onan-Cummins Certified