Thanks Olivier, I see that on the original schematics. I never had the inverter or the transformer, but I do see what you are saying. I installed a 4kVA inverter and switch it through a manual switch. That allows me to have 50hz power to the wall plugs and microwave when I am using US shore power (60hz) for the battery chargers and the AC units. I also installed an isolation transformer, which does the same function as the plug you are referencing through a dedicated receptacle on the transom, and it also isolates the ground to prevent galvanic corrosion from faulty shore power supplies.
Jamie