I would guess that the governor on your generator is causing the problems. When under load it is supposed to increase the power so that proper voltage (120) and frequency (60 cycles) is maintained at all times. They make an inexpensive (under $20) meter called a "Kill-a-Watt" that will tell you the exact voltage and frequency while the generator is running, especially when it is under load. These are available online or at Home Depot stores.
As an addition, I would guess that when the OP says the power comes back to the coach after the AC's are turned off indicates that the power is actually coming from the batteries/inverter, rather than the generator.