Asking the right question about generator&battery charger

There's no inherent benefit to an external charger plugged directly into a generator. Electrically plugging a charger directly in is the same as plugging the RV in with it's internal charger connected.
It all depends on which RV converter and which external charger we're talking about. Personally, if the one in the RV isn't multi-stage or doesn't have a high enough output I would just upgrade it instead of spending money on an external charger.