It must be a pretty old building.
It is a long shot, but take the cover off the outlet and see if there is a ground wire in there. If so, you could replace the outlet with a grounded outlet and ground the system that way.
One word of warning: Assuming you are in the US, the standard outlet is rated for only 15A and the 'special' outlet with one of the openings like a sideways T is only 20A.
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If you have 2000W inverter, a 20A will probably be OK, but this is undersized for anything over 2400W.