I am not sure that on a Allison tranny the reverse lights are controlled by a switch in the tranny itself. I believe it is controlled by the touchpad shifter and the attached computer. It would be very unusual for that to fail but not impossible. Not trying to insult you here but have you checked both bulbs? Bulbs normally age out at about the same time so it is not impossible for them to both be bad. Also there is probably a fuse for them.
"Gunner" USN Retired, Airdale
2004 Monaco Signature 44' Conquest. Detroit 60