Do you use white=negative, black=positive for DC in your RV?

If I see red and black, I know the black is neg. If I see black and white, I know black is pos. If you can't see the second wire and you only see a black wire---oops. Need a meter, find a ground, see if the black is pos or not.

I like to put some red tape on a black when the black is positive going to a battery post.