RV Generators can charge the RV house batteries when shore power is not available through the onboard converter or using a multi-stage charger.
The onboard converter may charge the batteries provided other appliances are not using the generator power. Different converters charge at different rates. Some can take a long time to charge the battery this way and may not fully charge the battery in less than a day, especially if the battery is below 50 percent. Another alternative for battery charging uses a multi-stage battery charger running off the RV generator.
Just like every other appliance in your RV, it is important to know how much power the charger requires. Each appliance in use adds to the load on the RV generator which has an upper limit specific to each model. If the generator rating equals 4000 watts, a 1200-watt charger leaves 2800 watts for other uses.
RV Generators for Power on the Road