So I looked inside the space where the second battery would go. I see a relay
 wit a metal exterior, with 2 red wires connected to it. Those two wires go to
 a small fuse box screwed to the wall, and then the wire goes behind the
 cabinet and goes into the wall and disappears there. Other wires coming from a
 hole iat the bottom are connected to the relay, and that's it.

So what am I seeing here? What is the relay for, what is the fuse box for?

And more importantly, where do I connect the dual battery kit and the second
 battery?

Thanks in advance!

Philippe