A 2 or 3 volt change while running a 15 amp appliance is not that unusual. When I was parked on a ranch, I would go from 120 volts to 108 when I ran the microwave. In the summer I needed a voltage booster to run the A/C in hot weather.
Don't worry about it so much. Only worry about it dropping below 108 volts while running the A/C then keep an eye on it. At 105 or less, I would recommend shutting off the A/C until it gets warmer or you buy a Hughes Autoformer, that will automatically boost the voltage if it falls below 110 volts by 12 volts.
Fred.