Freightliner Park Smart Apu Heat works for 30 min after truck shut off then quits

You probably have the diesel fueled bunk heater. That's what my truck has, it heat the coolant then you get heat. It's probably not firing up and burning diesel. They get clogged with soot and need to cleaned or replaced every couple years. They also make bunk heater the just blow hot air and doesn't heat coolant. Same thing they need to be rebuilt or replaced after a couple years.

 

On mine, I have to start the diesel bunk heater separately. The park smart controls won’t turn it on. I have a separate control for the espar bunk heater.

 

It might not be much help now, but if you have the coolant heater. I can tell if it's working or heating up because it keeps the coolant at about 160 on the dash gauge.

My truck is 2020 with the 8 batteries to run diesel coolant heater. It also has electric a/c in sleeper cabinet for summer. We just turn on optimized idle and turn fan on and set temperature dial. The thermostat will run heater or a/c automatically.

You could try setting optimized idle on the dash to Interior Comfort Mode On. This will makes the engine start for heat or a/c. It will let you know if the thermostat in sleeper is working. If the engine starts to heat sleeper then the heater is definitely not firing up.