So it sounds like the inverter was not involved in the problem, since its output was not connected to anything. That's a good start.
I would start by disconnecting the inverter, both input and output. The manual then suggests that you disconnect the battery from the converter, plug the TM into shore power, confirm that there is 120 VAC on the converter input, and then look for 13.6 VDC on the converter output. If it is there, all seems well. If not, there are a couple fuses inside the converter - reverse battery fuses. Make sure they are good.
You need a WFCO manual, not the glamorous advertising sort of info. You will find a good WFCO 8900-series manual here, which includes trouble-shooting steps and pictures of the associated test points and fuses. If you have not read through it, you probably should. The checks are simple.
https://wfcoelectronics.com/wp-conte...ies-Manual.pdf
Good luck - let us know. And if the generator is producing 300 VAC, it is probably time to trash it. There are many possible replacements. Something like a Honda EU1000 would be a good choice.
Bill