In all these scenarios, wouldn't the same current be flowing through the positive wire to the camper battery, and therefore be protected by the circuit breakers on that wire? The only reason you would need a separate breaker on the negative would be in the case where you could have an (excessive) current flowing on the negative wire which is not also flowing on the positive wire. I can't think of any scenarios where this would be the case.
Not trying to be argumentative here, but trying to make sure we are giving good advice. A breaker on the negative for a dual battery is not something I have ever run across before, so I want to make sure it actually serves a purpose and that I am not missing something.
It's OK if you don't get it. "We" are not giving advice. I'm giving my advice, and you're giving yours. You are, indeed, being argumentative. Stating that you're "not trying to be" doesn't have any effect on that. That's OK, too.