Not sure what jim is talking about.
All you have to do is run a positive from your main battery to your 2nd battery. Then you can run a negative from your main to your back battery if you want, or just ground off the back battery to a steady ground on your vehicle.+ to + and - to -.
Isolaters are not used in this situation. You can have a wet cell and dry cell combination too. But in my case I used a dry cell battery in the back of my truck, since that is gobs less secure, and I don't want to have to worry about my battery tipping over and leaking acid everywhere.