Does it have to be on-board? Some people with a class-B use something like a Honda EU1000i or EU2200i (which can run a 13,000 BTU A/C with soft-start).
The inverter generator will be more fuel efficient at low loads than the Onan. Of course, at the expense of convenience, you have to set it up and fuel it yourself, you can't just power it up and go like an auto start onboard generator. There's also the added advantage of being able to set it up farther away from your RV to reduce noise and Carbon Monoxide.