It isn’t always possible to connect directly to a power source, for one reason or another. Some of the equipment you carry may not be able to take power from a DC source, or for that matter, an AC source. That is why you need to carry a power converter or two.
Bigger units can be used to convert power so that you can charge your batteries. Smaller ones can take power from your batteries and charge or run appliances. Either way, you can’t make the connection without a converter.
As each RV or camper has different needs, it is best to consult with the experts to find out how large a unit you need to safely supply your requirements.