Jerry,
Remove the AC access cover on the side of the Magnum and check the input AC and output AC (pass-through).
If there is input voltage and no output voltage, then the invertor is the problem.
If there is no input voltage, then the problem is upstream of the invertor (main breaker panel or the wiring to the invertor).
If there is both input and output voltage, then the problem is downstream of the invertor (invertor sub-panel or wiring to it).
Otherwise, it looks like you covered all the bases.
FWIW, this winter we had nearly the exact issue with our PT. I was running a space heater (and I knew better), lost all pass-through AC and charger functions. I didn’t want to tax the batteries, but the invertor function seemed to work.
The initial err-code was over-voltage then became an internal fault err.
I went through the troubleshooting steps with several resets; the inverter would work for a few minutes then err-out.
It took several attempts, but I finally got through to Magnum tech support. The tech was very thorough and patient, not at all in a hurry.
We repeated all the steps that I had already performed including a hard reset.
His diagnosis was a faulty control board but he was concerned that something else was going on…didn’t want to start throwing parts at it. He suggested a bench test. I took the invertor to an authorize repair facility where they diagnosed it as a bad control board and a bad AC board.
It was about $770 for the repair. I had to find the control board myself as they were extremely scarce, none were available from Magnum.
I can’t help but feel that the damage may have been self-inflicted by running the space heater through the invertor (pass-through, not inverted).
Control board $330
AC board and labor $440
-Scott