Had a similar problem several years ago. The AC main breaker was 18-years old. I could not figure out why the breaker was tripping because it seemed to test well. Then I looked a little more closely at the 30 amp AC lead from the shore power connector. The AC power lead was in a wire bundle wrapped in spiral wrap and was not easily visible.
The attached picture was that AC power lead where it connected to the back of the circuit breaker. There was a lot of
corrosion on the filaments and they were developing high resistance and
heating up when, during the
winter, we plugged in a 1500 watt
electric heater and ran the 750 watt
water heater while doing the
winter evening
liveaboard things. I measured 2.5 Kw thru the lead after I replaced all the
wiring between the shore power connection and the AC breaker. That is a lot of heat!
I was very lucky the AC panel had not burst into flames.