It takes a lot of current to start the generator or the engine. Running either for just a few minutes will not fully recharge the battery. Perhaps the "low battery" alarm is just doing it's job. Measure the battery voltage when the alarm starts beeping. If it's fully charged it should read about 12.6v if it's not being charged. If it's less than 12.0v, the battery is discharged.
If you keep the RV plugged in, the converter should recharge the battery. When being charged, the battery should read 13.2v to 14.5v depending on your converter and the battery state of charge
