Fan Cycling Problem.

[Chamberpains]

Not that this is your problem or a cause of your problem. But I have seen AC systems so overcharged that they cause these kind of symptoms. Most commercial trucks only hold 3.5 to 4 lb of freon to operate the system properly. I've recovered trucks that had upwards of 12 pounds in them. This causes the highpressure switch to go nuts on its own because there is just way too much liquid freon in the system. Most of the time it's from drivers buying cans of freon and just dumping huge amounts of it into the system thinking the more you have the colder it gets. When in reality it stops functioning after it is overcharged. The AC will not blow cold and the compressor will not engage. If you feel you or someone may have done this to your truck you may want to go have it recovered vacuumed and recharged to proper levels.
And I doubt that this is still a problem but who knows anymore. We used to see the occasional truck come in with propane in the AC system. Mainly down around Mexico or trucks that travel to Mexico. Propane's a good refrigerant and is still used in a lot of mini fridges and bar refrigerators. But banned in vehicles. Too many problems with explosions and high pressure issues.