Water pump lights etc...use battery power, your generator is powering the converter, let it charge the batteries before you turn on the A/C
Using the extension cord will increase voltage drop and breaker trips
Having other items using power
Letting the A/C cycle the compressor will increase the chances of breaker trips
As the A/C compressor warms up, and ambient temp warms up, the start surge for the compressor takes more amps
This is the most likely problem, having the A/C on a setting where it cycles the compressor
In a mild day it might cycle without overload
On a day that gets hot, you reach a critical point where the restart cycle load is too much for the generator