Based on BTU equivalence to gasoline, I would estimate 8-10 hours at half-load on a 3.6-4 KW genset, which is typical for running an A/C and normal loads on a 40-55 Amp 12V converter. Most RV gensets are sized to handle peak loads for the 120V hookup (thus 3.6-4 KW for 30 amp service) and normal loads are about half peak.
Peak loads come in when trying to use multiple heating appliances like hair dryers, toasters, electric fry pans and griddles, and heavy motor loads like vacuum cleaners, any of which is a likely overload when the A/C is also running.