Let me start with I am biased and would not take an Onan for free. Paying for parts is one thing, I refuse to own something where I get ripped off for parts. I had a nice Honda EV4000 and let it go with an RV I sold and have kicked myself ever since.
So I have my nice little Toyota set up with a 8000 btu window AC and run it with a Honda 2000. The Honda is in a sound box, basically an modified aluminum truck tool box. Could it be stolen, of course they could also take the whole RV!
There is one option for running An AC with small generators, I have not tried it yet so will not recommend. If needed I would spend my money on one!
https://www.microair.net/collections/easystart-soft-starters/products/easystart-364-3-ton-single-phase-soft-starter-for-air-conditioners?gclid=EAIaIQobChMI14Ko2q3K6AIVB2KGCh2kYwBoEAQYAiABEgJzTPD_BwE&variant=30176048267
They tell me Honda replacement parts are expensive, I do not know as in 20 years I have never bought a replacement part. Except for a pull cord and that was generic!