my APU is not blowing hot air

Check in the frame of the truck for shut off valves. Also make sure you main panel is set to heat mode. Possible that the heater unit does not ignite?

 

I don't think APU's are set up well for the "Heat" option. Every truck I have ever been in that has an APU, also has an Espar heating system (fan on the back of the sleeper, and Espar unit under the bed).
APU's are better served for air conditioning, and AC power.

You do NOT need the APU to run the Espar, but you still use the control panel. Set the button on the left to 'Heat', and do not turn on the APU with the button on the right. You can still control the fan, and the thermostat....but to get the Espar to blow heat, the APU must be off.

 

i f you dont have an espar, just plug in a cheap 30 dollar electric heater to the Apu

 

I was going to say the same thing but he said it worked good before?

Big Chevy every heater unit I've seen is independent from the APU. I believe there is a way to integrate them but I never seen a company use that option. They usually have an independent dial and they take a good 5-10 minutes to start blowing. The igniter will light the fuel and only when the unit gets burning good then the fan will start to blow. It will blow out by your feet and not the vents. The fan won't come on right away and many new drivers think it's broke. Turn the dial and wait, wait, wait.

A picture of your bunk controls would speak a 1000 words.

 

It might help if we knew what brand APU system you have?