Depends on the AC. My Duo-Therm paperwork recommends a 3500W generator. It's about 15A continuous and 19+A at start. This recommendation recognizes that other items like the refrigerator and converter can also be continously drawing generator power, as much as 6-8A, that generator power decreases with altitude, and that you don't want to operate a generator continously at more than about 80% of its continous capacity if you want long life from it.
Duo-Therm DOES have a high-efficiency unit that only draws 10A continuous, and both can simultaneously operate on a 30A hookup even with the converter running. A friend of mine has a pair of these on a 30A Winnebago, but I don't think Airstream uses them.
This common 3500W recommendation is one reason Yamaha has recently one-upped the Honda EU3000 (2800W continuous) with an almost identical model (EF3000iSEB) that can use its starting battery to surge to 3500W, rather than only 3000W.
Keep in mind that naturally aspirated (non supercharged) engine power falls off with altitude. Carbureted engines are even worse than EFI engines at altitude because the mixture goes too rich and power also falls off from that. The rule of thumb with portable generators is that you lose 3.5% for every 1,000' above sea level.
Using that, a 20.8A continuous EF2800i is down to about 16.4A at 6000', and can only surge to about 18.4A there, where a pair of EU2000s at 26.6A continuous would only be down to 21A at 6000' and could surge to 26.3A at that altitude. Quite a difference! You get what you pay for.
I was interested in the EF2800i when it first came out, but didn't like several things about it. One, it weighs 83 lbs fueled where the EU2000s are only 55 lbs each fueled. Two, its gas cap vent doesn't shut off, so if you have to tilt it to get it into a compartment, etc, it dribbles gas. Three, it's 7db louder than a PAIR of EU2000s, meaning it has to be located about 50% further away to be as quiet. And finally, an owner of one of these told me he had to unplug the converter when in the mountains, or the AC would start slowly (not good for its health).