APU battery powered not working

Check at the battery box first.
They may have disturbed or broken a connection to the battery?
Also could be an inline fuse to it at the battery that might be an issue?

 

what type of apu? Carrier ComfortPro or Thermo King tripac evolution? Makes a difference on how things are setup and where to start.

 

Trace the wire that's supposed to have 12 volt on it back until you find the issue.

 

It's not that simple. The power is supplied through a circuit board either in the apu outside or under the bunk in a fuse box attached to the evaporator case. If it's a TK, it's pretty simple. If it's a Carrier, it's not so simple. They are night and day different. TK uses automotive technology for cooling and Carrier uses a heat pump setup - mini version of what you use in your house. It runs off a generator that supplies 120v to the ac compressor. TK uses an automotive belt driven ac compressor.

Both use a circuit board to supply the power but the carrier unit is much more in depth with how they tie into the truck.

Jumping the truck could have fried the circuit board.... just like it is possible to blow up a battery or fry the ecm when jumping or welding on the truck.
As I said, the little tiny wire going into the thermostat control is not as simple as tracing the wire. Most likely, blown fuse.

 

And each have easy ways of troubleshooting the problem... the carrier unit has a reset button on it that could have tripped. Similar to a gfi plug in your bathroom