How do I hook up the three batteries in this RV?

There are three batteries in a line. the left battery powers the RVs Engine, and is rechargeable. the two off to the right look like they were supposed to be in an array or something, the far left (+) terminal has a breaker installed (like a retro flip metal bar to other side thing) and goes somewhere into the engine. then theres another red line coming out of the engine that looks like itd fit in the middle batteries red terminal but theres no ground for these batteries, and two black wires about 3 feet long that go in somewhere. its an 85 GMC Pinnacle R.V.