Two 12V batteries - can't figure wires

Hi guys.

I've read the threads about connecting two batteries by linking the positive and negative connections, but on my Trailmanor I can't tell what's what. The left-hand battery has three black cables - so which goes where?

I have power when I'm plugged in to the house, but not when I try to run on battery power. When I press the "test"button inside the Trailmanor, it says that the battery is fully charged.

The batteries are clearly not connected correctly.

1. The white wire on the right battery is a ground, not a negative wire - right? Is that all this battery needs, a ground and a connection to positive?
2. The left battery wires include a positive wire (red one), but there's no color difference in the other two that lets me know if it's supposed to be a positive or negative.
3. The fuse blew out...

By the way, it's a 2007 2720.

Thanks a lot for any help you can offer connecting these cables correctly. My wife and I bought this unit used a week ago and are hoping to take the kids camping over X-mas (we live in Southern California).

Got my fingers crossed...