With his suggestion for 30K ohm or less measurements, and your thoughts about the neutral side I "stand a chance" to find it. Next visit to the boat I'll try powering off the battery charger circuit and try a load on the hot water heater circuit. If my problem is only one circuit I know it's the load side, otherwise it might be the buss behind the breakers or perhaps the shore power wiring before it gets to the breaker.

I know it's not the devices I'm plugging in, because I tried several different devices as loads. Will get back to it this weekend if time permits. I've got all winter to figure this one out.