SINCE none of the ones you are thinking of is a Cummins repair facility I would not expect them to be able to advise you on the powertrain.
For the rest of the coach the RV Specialists are the ones I would use. Inspection only companies are worthless as there is no warranty provided in case they miss something and there is no nationally recognized standard for training or certification. So basically it is a give them x amount of money and if they notice something they will tell you and if they miss something important you have no recourse
Find a good Cummins repair place to look over the powertrain and also get an oil analysis
Traveling with my best friend my wife!
