If you put a softstart device on one of those air conditioners a 2200-2500w unit should be sufficient. The only real caveats are to get a quiet inverter unit, and if you're running the a/c you're probably not going to be able to run any major appliances at the same time. Of course a 3600w unit would replace what you lost but they're rather heavy.
There's a 4000w installed unit available to replace the Onan for significantly less money if you're interested.
https://rvmp.co/products/flex-power-4000i-rv-generator