I'll "generalize" this discussion a bit: Go with a built-in generator powered by the same fuel as is used by the chassis engine - and of course make sure this built-in generator gets it's fuel from the main engine's large fuel tank.
I just plain don't get buying a motorhome without making sure it's design and systems allow for as long as necessary built-in generator runs with little excess generator noise, exhaust odor, or too-small of a built-in fuel supply. For example, we just completed a short but wonderful trip with the grandkids that offered us an opportunity to drycamp overnight during a hot, but otherwise superb visit to Antelope Island on the way to Wyoming. To keep from frying due to a July afternoon arrival at our campsite on the island, we merely fired up the built-in generator powered by the main engine's fuel and ran the A/C while we rested until it cooled off outside. To us, this is a far more valuable use of the built-in generator than for running the microwave or any other 120V appliance.
It's priceless to have the flexibility in one's Class C to be able to do this. Restricting our RV trips to drycamping at cooler high altitudes or to non-hot months would never do it for us. We spent too much for an RV to have to use it while placing this limitation on ourselves. We start another multi-week RV trip this coming August and will count on drycamping at times with no lapse in A/C comfort - regardless of where we camp. For batttery charging we can use the main engine's alternator initially, then follow up with the built-in generator or a small portable generator.