Excellent response to my question because it does help define a better answer.
The reason I am interested is to run dc things without running my starting battery down.
I would use an isolator with a dual switch. If I want to run game, fan, or what ever while I am camping.
When we were out in May, my son and his buddy ran my battery down playing games and music. I replaced my
battery shortly after that as is was over 50,000 miles. It was due. But I want a backup battery to run options.