The latest information I have on this fire is that the man was using an electric heater, powered from an inverter, powered from an oversize alternator powered by engine idling.
I'm not sure what component actually failed, but since he said the fire started in the engine, I'm thinking it was either the alternator itself or the wiring. It takes a lot of amps to run a heater, and over time that gets dangerous. If it was a cheap inverter, that's also suspect. I've seen cheapo inverters get hot running the smallest of loads.
I don't know if anyone here is doing that, but I wouldn't recommend it.