Need some advice on the Allison 6 speed in my 96 Motor Home. Tranny was overfilled and when I detected it had the filters changed and new fluid put in at a local diesel truck repair place. Tranny had been working fine.

When I got it back, it wont shift to 6th (other shift spots seem fine) and the trans shift read out now says "service".

Nearest Allison dealer is 100 miles from here and they are of no use on phone. They wont even venture an opinion as to whether it's safe to drive to them.

Any ideas?