Well, when I first saw your picture of the battery bay wiring I thought OMG I would hate to trace something in there......the diagram is much better.
The DC current feeding the inverter takes the shortest route from the batteries. So the ground side is obvious......and the +12v side of the inverter passes through the inverter fuse and goes directly to the +12v coach battery terminal. So that is the path to be checking for bad connections IMO. The double termination on the inverter fuse just ties the +12v coach battery to the rest of the coach loads.
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