We had a similar problem with our older coach. I bought an aftermarket GPS speedometer / odometer. Was a simple installation. Its a more expensive unit due to its accuracy, all GPS aftermarket speedo's arent the same. If you want the exact model post and Ill retrieve the data. FYI ours works perfectly and solved our "cable from the tranny" issue.
1993-37' Holiday Rambler Imperial
2018 motor, radiator, much more