The OP's rant only goes to show he has an amazing grasp of the obvious. Anything wired in parallel is obviously inseparable from the others. And I have yet to come across anyone who believes otherwise. Some sort of isolation is required to, well, you know, isolate them.
I had a math teacher, in high school, that when a student asked "Can I go to the bathroom", he would reply, "Yes, you can". But when the student would get up from his desk, the teacher would yell "WHERE DO YOU THINK YOU"RE GOING?", to which the student would reply "But, you said I can go to the bathroom." The teacher would then point out that the student had asked if he was capable of going to the bathroom; but if it's permission he seeks, the proper question is "May I go to the bathroom."
So the question is not whether or not a converter CAN charge a battery, but rather WILL it do so without energizing the system, while paralleled into the 12v system? Well, dah! While I do understand the context within which the OP made point 3, he puts forth some absolutes that leave no room for individual wiring designs. He assumes ALL rvs are wired identically. I guess I just don't see the need for the rant.
Has anyone here ever come across a manufacturer claiming their converter is capable of internally isolating external loads, when, in fact, it wasn't?