OK...

- you've replaced the PTC.
- cleaned the contact tips.
- have a new battery (assuming one just like what was in it, NOT a battery pack).
- verified there are not other devices connected to the load side going to the controller.

now thinking outside the box...
- is the PTC polarity sensitive?
- is it still getting hot?
- are all connections in the circuit tight and free of corrosion?
- are the contact tips actually closing? (try a new contactor)?
- maybe line contactor is just closing too slow (try a new contactor)?
- check the negative side of the circuit for bad connections or corrosion

the ultimate problem may be in the controller itself not detecting the voltage correctly. (I'm reaching here... just a radical thought) ;o) I would only consider this only after verifying all connections and voltages, resistances have been checked at all connection points around the complete circuit.

one thing i do know about these new AC systems
they are extremely sensitive to voltages so even an incorrect battery can cause problems like this, the battery has to be the exact type, size , voltage and amp rating designed for the truck, battery packs will not work correctly.
one other thing I've ran across alot is even though the line contactor appears to be working it could be slow or not closing completely, simply replacing the contactor assembly remedied the problem.

just some thoughts that come to mind, i'm sure there are some i haven't thought of someone else may be able to assist with also.