Don't forget about the solenoids..
On the curbside, you will find a box likely in a panel on the upper right hand side. Usually the middle solenoid is the first to go; there are little bimetal connections made inside of the silver solenoid, I think it is # 28. A good and simple test is to have someone run the OTR for a couple of seconds while you check the feed side amperage and then the output amperage. It should be putting about about 100 amps when the switch for the AC is on. If not, the fix is to simply replace the solenoid, about a 15 minute job.
Or you can go to a Prevost Car facility at about $2000/day and have them fix it.