Anyone with a Cummins 5.9 commonrail and Allison 1000

Talk to the Allison people. Whether you do or do not want the 6th gear depends on your weight and your engine to ground gear ratio. The engine to ground ratio is dependent on your differential gearing and your tire size. I retrofitted a Duramax diesel and an Allison 1000 from a 3/4 tn pickup to my coach but it is only 15,000 lbs and my gearing matched the pickup. With your numbers, Allison can change the shift points to suit your setup.