I had a similar fire O&C a few years back. Same senerio, low mileage, a unit had been installed by another manufacturer on a bare chassis. Same statement from the unit manufacturer that there was no "connection" between the chassis and added on unit. However during my research /investigation, I found that in order to power the unit a modified "shunt" was intalled on the starter. The wiring between the starter and alternator overheated due to a misfit cable on the shunt.
Request ALL wiring schematics,locate am exemplar unit and do an O&C.