Originally Posted by Biljol
That “extra ground” is the main ground for the house 12v system.
When I replaced mine (to a magnum) that ground needed to be connected to the negative lug on the magnum. I believe you will need to do the same and connect to the negative lug on the inverter.
Thanks for the destination / confirmation of the 'extra ground'
(When I removed the inverter - I 'lost' a load of electricals until I reconnected the 'extra ground' to the temporarily redundant battery ground - so suspected something similar just hadn't got around to investigating)
Hope I have enough slack to reach the negative stud on the new inverter if necessary - time will tell