We've used the built-in generator mostly for cooling down the house section while traveling in sunny hot climates. We do this when the house is occupied for travel (e.g. carrying family) or to pre-cool on the way to the campground on very hot days. We've also used it during enroute stops, even when not the C is not occupied, to keep the A/C running. In full sunlight, with outside temperatures in the 80s or higher, the interior can get to over 100 F in an hour or less.
We've not used it this way traveling in Michigan, Wisconsin, northern Minnesota, not even in summer. We've found we get adequate cooling with use of the A/C in the cab or often just vents and windows.
We've used the generator in a campground just once, when a lightning strike on a nearby substation took out power during an early morning thunderstorm. That required going out in the storm to unplug from shore power and plug into the generator socket (no automatic transfer switch on my C).
I've never run the generator while sleeping, whether or not I would have liked to have air conditioning. I don't want to sleep with CO building up underneath and around me.
With all these uses, I've put fewer than 200 hours on the generator, in 10 years of RV use and 30,000 miles of travel.
One alternative to a generator for on the road use would be a whole-house inverter of 3KW or more, and at least 300 amps of 100% duty cycle alternator capacity (the equipment supplied for emergency vehicle use on these van cutaway chassis).