Your batteries are all wired in parallel. This means all 3 are used.
2 of them appear to be of one model, and the 3rd of a different model. It is usually discouraged to mix battery models. It is definitely discouraged to mix batteries purchased at different times.
I suspect the red button is a circuit breaker rated at 105 amps.
My best guess is that the circuit breaker either feeds the body electrical, or feeds the A/C units. The way to find out is to disconnect that wire and see if everything inside the bus still works right. If everything seems to work, you should be able to leave that wire off. I would tape it up though, just in case that extra alternator you have is attached to it.
My battery compartment looks to be the same size as yours. However, my batteries would eat yours for breakfast. There are pics here: http://gallery.giffords.net/v/thomas/batteries/
. Here's the best one:
I *think* mine are 8D sized. They're really huge. I think it would take 2 people to carry one of these, or someone that doesn't mind destroying their back. In any case, they've *never* failed to start that engine in less than 2 seconds. Even at temps around 15F.
In your case, I think you could replace your batteries with a pair of decent diesel starting batteries. The 8D size probably aren't cheap. However, a place like NAPA or Sears should have the size that are used in the diesel pickup trucks, and a pair of those would probably work for you.
Anyway, I hope this helps.