Morningstar, as Flying Cloud indicated the load imbalance that trips an ELCI is a differential in current
flowing in the Hot and Neutral wires of your shore power AC circuit. All current
going in from the Hot should leave the circuit via the Neutral. As he stated if one of your devices has some inductance I suppose it might be possible to get more current flowing in the Neutral than Hot and vice versa. Nonetheless, I am not sure there is anything you can do about that other than change out the offending device (if you can isolate it) or pull the wires from the toroid to defeat the ELCI. Others likely know more about that than I do. Intermittent problems are inherently very difficult to isolate.
Another potential issue, one the ELCI was designed to protect, is current leakage from Hot to a local Ground. Not a direct short which would trip the standard breaker, but enough current leakage to electrify a metal enclosure or the water
around your boat etc. In this case some of the current flowing in through the Hot would leave via Ground instead of the Neutral wire and there would be more current in the Hot wire. This condition will trip the ELCI. Just as you have experienced, I had intermittent ELCI tripping. I had an AC outlet in my galley
mounted on the super structure of an exterior wall (trawler). In this location there was a chase for air flow to a blower in the galley
. For some reason the outlet was not mounted in an enclosed box plastic box and the back side of the outlet was exposed to airflow and moisture in the chase (maybe ABYC does not require an enclosure). Depending on precipitation, humidity, wind
etc. I was seeing intermittent tripping of the shore power ELCI. This of course never happened when I was there. I stumbled upon the way this outlet was mounted while replacing the galley blower with a much quieter version. I ended up moving the outlet and placing it in a sealed enclosure to eliminate the potential for moist air flow over the outlet. My theory was that the moisture on the outlet created a condition where there was enough conductance to trip the ELCI, but not enough to blow the breaker or the GFCI. This theory could be complete BS, but I believe I did see a decrease in the frequency of ELCI tripping.
My guess is that the ELCI protection is a good thing, but every time another monitoring / safety
device is added it is certain to cause headaches. Good luck.