The stock converter in an A122 is rated at 30 amps output, 12V DC (nominal). That would be 360 watts max output. Allow for losses in conversion, max input is less than 450 watts. There should be a plate on the converter giving the actual 120 volt input (maximum) in either watts or amps.
The converter input is going to vary considerably depending on battery charge level and other 12V loads. When the converter fan is running, power draw is more than 100 watts (a realistic guess based on loads in my A122). If the converter fan is not running, it's drawing less than 100 watts. Very seldom does my converter fan run - when it does the battery is very low and/or the fridge is on 12V DC.
A 600 watt generator will power your converter and recharge a laptop from a 120 volt plug.
As stated in the A-frame forum, a 2000 watt generator will power the Cool Cat A/C (around 1400 watts) plus run the converter plus small loads.
The generator output will decrease with altitude, but the loads do not. A 25% loss at 6,000 ft (Lake Tahoe, Colorado Springs) is normal for a carbureted engine.
2014 Rockwood A122