I carry a 1000 watt Honda for battery charging only. Have done so for the past 15 years. Works very well.
I also like having the Honda along with me as a "spare tire" so to speak. If I were to accidently draw down my starting battery in any way (eg leaving the headlights on), my Honda is there to save the day.
Along with my faithful little Honda I have a Black & Decker 45A smart charger and a gallon of gasoline.
I won't leave home without them.
I do not use the RV for charging batteries except when I'm going somewhere in it anyway. Making the RV idle just to charge batteries can cause engine damage relating to excessive idling, something best avoided.