No power except microwave when plugged into shore power

To remove just the left cable I would not disconnect the batteries. Just make sure you do not hit the metal mounting bracket while disconnecting. If you need to connect it to the right connection then yes I would disconnect both negative terminals on the batteries because on the right side it's easier to hit something if you are not careful. I would not disconnect the batteries but I work on stuff like this all of the time and learned a long time ago how hot a wrench can get when you short it.