Any size generator or solar array will eventually charge them. Your question is probably more like how long will it take. If you run a 250 AH bank down 50% that's 125 AH. If you have a 30 amp charger it will take five or more hours to charge and use around 600 watts from your generator. Or, you could get a 60 amp charger and do it in about 3 hours while using 1200 watts from your generator. (Battery chargers are not 100% efficient, nothing is.)
The 12 volt output on generators is worthless and shouldn't be used. It is generally unregulated and very low amperage. If you read the fine print it's probably something like 8 amps or so where your RV's built in converter (charger) is probably 30 to 50 amps.