2012 Prostar AC blows cold only on HOT

Dig the ambient air temp sensor out from the harness, driver side, under radiator. Move the sensor up to the windshield wiper cowl. The sensor has a clip on it, clip it to the cowl up by the drivers door top hinge, get it into the open air, or as close as possible. Then, evacuate the AC system, refill with straight refrigerant, no dye. Also insure everything is working properly, and the only way to do that is with Diamond Logic, if you do not have a copy of that, a trip to the International dealer is in order.

If you evacuate, I would change the dryer can and the expansion valve. The expansion valve, only use the OEM valve, do not use an aftermarket valve. The other AC parts it is ok to use aftermarket stuff. I use these guys, Heavy Duty Truck & Off Road AC Parts from Trucker AC

 

Or the blend door actuator is bad...

 

Could be, there are three of them, two under the dash, on left side of AC fan box. They are the same actuators used in the 1999 Jeep Cherokee I believe. Dorman makes them, much cheaper than the dealer. The third is located on the firewall, you have to remove the windshield wiper cowl to get to it, it is on the right side of the intake box, towards the center of the truck. For some reason those rarely fail, where the ones under the dash do fail quite often, the sleeves break on them that control the doors. The two under the dash are different parts then the one on the firewall.

You should hear them open and close, if you cannot hear, replace. If you can get Diamond Logic on the system, it will show all of the faults, including faulty actuators. That is what I would do before you start throwing parts at this system. You can easily spend a grand and never correct the real issue. Best to just bite the bullet and get it properly diagnosed. You can do the repairs, the system is not that difficult to work on.