"So for charging, connect charger red lead to positive of battery who's positive is 12v lead to trailer and connect black charger lead to trailer ground where 2nd battery's negative lead connects to trailer. Thereby its one big battery. Is this right?"
Exactly right. If you charge with the converter in the trailer it's no change at all.
Just be sure you get the 6V hooked up properly. Every now and then someone switches from parallel to series batteries or vice versa and gets the wiring wrong.
Trailer positive lead +[battery]- +[battery]- trailer ground lead. That's what you want for 6v. I'm sure you know this already, just chipping in in case.
