If it's wired to the inverter it would be a 120VAC connector for a custom made extension cord, with the matching plug at one end and a std 15/20 amp cap at the other.
I would "suspect" that the P.O. used that odd-ball method of connecting to indicate that it was connected to the inverter
Same answer if it's connected to the "Converter", except for 12 VDC.
Best bet... get out the volt meter and find out what's on the pins. And shame on the dealer for not having a better answer.
BTW: That's a standard connector and matching plugs are easy to find. I think that they are also used as a trail light
connector instead of a flat 4 connector.