Basic battery question

[DanSTL]

Just bought a 2009 2720 and it only has one barely working group 31 deep cycle battery and an empty spot where battery #2 goes .

Problem-. Nothing except for electric tongue jack works on battery power, no lights or fans. Electric 30amp works fine.

Next? Not sure what these wires are for but looks like I need a 2nd battery and/or need to redo all connections to get lights and fans to work on battery?

Any YouTube video that would guide a newbie on what to do next?


Thanks