wow, there is a lot in here, I mostly skimmed over. my solution = 2 switches, both on one switch (Duplex Double Toggle Switch).
I put a 30 amp switch in where, on one side the inverter/charger is powered, and on the other, shore power, thus they can not both be on at the same time. a second switch is on / off for the existing charger. I ran the power line feeding the existing charger to the switch. when I use shore power, the inverter is off, and I switch the charger on. when I am on inverter, I switch charger off. both switches are side by side on the one switch and labeled so I don't forget. I am very happy with this and thought a clever solution if I do say so myself.
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