We own our Class C, so $3 per hour is not an issue on how much we use our built-in generator (we carry another small portable generator for battery charging) ... but here's some of the things that a generator can be used for if one wants to be ready for anything when on the road or camped using a truly self-contained RV:
1) Powering the microwave when drycamping.
2) Powering a small electric heater when drycamping in cool weather should the propane system fail or run out of propane.
3) Powering the refrigerator when drycamping should the propane system fail or run out of propane.
4) Charging the batteries when drycamping if one doesn't have solar or if the sun isn't shining enough if one should have solar.
5) Powering the entire 120V AC system of the RV when full hookup camping and the power should fail at the full hookup campground.
6) Powering a hair dryer on human hair or pet hair after washing when drycamping.
7) Powering an electric chain saw when trying to get to a boondocking campsite, or use the campsite once there - that may be overgrown with trees or brush.
8) Powering a full capacity - but relatively inexpensive - 120V AC tire inflation compressor packed along to address tire emergencies instead of having to buy a relatively expensive 12 volt full capacity tire inflation compressor.
9) Powering an electric lug nut wrench for emergency tire work when on the road or at a drycamping site.
10) Powering a full capacity outside fan to keep folks cool and the bugs away when wanting to party/eat/talk outside under the awning in hot weather.
11) And last, but not least, power the RV's rooftop air conditioner for short periods or long periods when driving, dryparking for a few minutes, or drycamping.