The only way to solve this is to do some sleuthing:
1)Clamp hot and then neutral from each circuit with a high quality sensitive ammeter and see if amps drawn = amps returned. Difference = leakage to ground, no difference = powered appliance consumption or faulty appliance.
2)Clamp hot from each circuit and one by one turn on/plug in the appliances/lights on that circuit. Check amp draw and voltage vs boilerplate or wattage and compare, small differences can add up.
"he has a 50+ year old house with MANY additional panels and circuits added" Does that necessarily mean more oportunity for ground leakage or just more legitimate consumption.
"monthly electric bill averages about $300 a month" assuming $150 of that $300 bill is excessive and cost of power is $0.12/kWh at 115V then you are looking for a total power consumption of 1.736 kW or 15 Amps, should be easy to find that.
