The question I would have has to do with the cable that connects the battery to the switch. Is this cable a primary cable with an in-line fuse or is it a smaller cable used just for charging with an in-line fuse? Since you are in the Carribean, I would check the connections of the switch, fuse and batteries for corrosion. any corrosion will increase the resistance in your wire and could potentially be your problem. Some of the older boats used non-tinned primary wire and if they were corroded, that could be a problem too, but if you just had all of that re-worked then....................
