Hard to be specific because those batteries may be on their last legs and have little available capacity. Also depends on what charging setup you have. Some rigs have charging from the engine alternator but that is often not good enough to get the batteries more than 3/4 charged after a long drive. Some rigs have fixed voltage converters rather than decent chargers so that also may limit how much charge you can get back in by running the generator for a couple of hours.
Best bet is to sort it all out before you leave home. Run the generator for two hours and then switch the fans and lights on and monitor the battery voltage using a digital multimeter over the next 12 hours. Then repeat the process by running the generator for another 2 hours and then fans and lights.
This will give you a very good idea of what to expect and whether your system can cope and how many hours a day you need to run the generator bearing in mind that the less hours you run the generator, the more friendly your neighbours will be.
Running a 12V fan might be slightly kinder to the batteries because usual inverters drag from 1 to 2 amps when they are sitting there doing nothing but again, any comparison depends on the exact types of fans you use.
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RVs. USA - Airstream Cutter; in Australia - MC8 40' DIY Coach conversion & OKA 4x4 MH; in Germany - Hobby Class C; in S America - F350 with 2500 10.6 Bigfoot camper