Finally got this installed and seems to work ok. Sits in-between the ProgInd EMS-LCHW30A relay and the 120V distribution panel as the ProgInd EMS didn't like the floating inverter output.
The relay (NTE R04-11A30-120) has relatively small screws for attaching big wires, so I added copper lugs and mounted it in an 8"x8"x4" plastic box so there was room to manipulate those big wires.
And wired it so the spring-actuated NC terminals go to the inverter and the relay actuated NO terminals go to shore power. I think this is opposite to how BestConverter's transfer switch does it. But it makes more sense to me after Mex's explanation above. And the ProgInd EMS relay also uses the NO terminals for shore power.
So I now have 120V shore power running through *two* coil-activated NO relay contacts to get to the distribution panel. Hopefully that's not a problem.
As for cost, I didn't save any money doing it this way. The stupid box and copper lugs were not cheap. But I learned a few things (Cogito ergo sum).
