Which isWhivh

Just crank the engine for a couple seconds then shut off if it started. Go measure the battery voltages. The low one is the engine battery.

Disconnect a negative terminal and see what you loose. Either the engine won't start or you'll loose interior overhead lights. You'll need to unplug from shore power otherwise the AC to DC convertor/battery charger will step in and keep the house alive.

Can you check the battery voltages under various conditions? If two of them show 13+ volts while plugged into shore power but the other is sitting at 12.xV, then the two are house batteries connected to the converter and the leftover the chassis battery. The starting battery also would show a voltage drop (hopefully not a huge one) when cranking the engine, while the house batteries wouldn't.

Ok, great. I does kind of look like it, lol, but a little confirmation never hurts!

Just guessing here...

If all three are the same (all 12v) them I'm guessing the one is meant to be your chassis/engine battery and the other two hooked up in parrallel for the coach/house batteries.

Again, just guessing.

A couple of pics of the batteries and the battery compartment would give us a better idea of what you have. Also, you may want to try following the battery cables and get an idea of where they go to.

No, all 3 are the same. I'm mostly wondering which one is for starting the engine?

Are the batteries different?
Such as the 2 on the bottom say deep cycle and the single says something different?

Usually the single battery is for the chassis (starting, headlights etc)
the two would be for the house part (furnace, fridge, interior lights etc)
But your could have been modified