AC UNIT

Blend door is my guess. Test the actuators, remove them and manually test the ease of each door..one may be binding up.

Got the same issue with my 2023 389..blend door in cab not working properly..actuator isn’t strong enough to move one of the doors..

You got the same deal in your SB

 

Hot/cold valve or switch

 

No it doesn’t.
Mine actually has its own heater core and ac condenser? In the bunk.

I probably shouldn’t be trying to compare my truck to yours, I’m sure things have changed over the years. To me it just sounds like a blend do door not closing.

I recently woke up to a nice cool sleeper and when I opened the curtain it was 200 degrees in the cab, lol

 

Some Peterbilts have a solenoid that lets Freon through the rear EVAP and expansion valve. It’s controlled through the bunk heat A/C switch. I’ve got 2 the exact same year model. Both 379 one has it and one doesn’t

 

One thing you can do is a pinch off plier and pinch the heater hose. That way you know if it the heater valve or blend door. It also can be low on freon, hook up the a/c gauges and check it.