So if the second battery gets its charge from the first battery what charges the first one? Your alternator. Your factory alternator is only meant to provide power for the vehicle ans its stock components. If you are at the point where you are adding batteries you need yo look into upgrading your alternator as well. It takes power to make power. Just adding batteries is not the answer as your alt will still have to charge them. The multiple batteries just give you a bigger reserve. Meaning when you start getting into voltage drop it's going to take you alot longer to get back to a properly topped off battery meaning your alternator is going to have to work longer not necessarily harder as its only capable of doing what it was built to do.
You give alot of marginal advice Tim. From someone who didn't know you should fuse both ends of a power run from front to back its really hard to even take you seriously