X2 on needing acid. Lead acid batteries require lead and acid. The water alone won't do it. When you add the distilled water you are actually just replacing water that has been evaporated off and rediluting the acid while making sure the plates remain covered.
I have heard abouf flushing the batteries out, but I think that is probably some sort of an oldtimer myth or legend. I would think that tipping the batteries upside down and the associated shaking would do more to DAMAGE the plates than help anything. If the batteries are old and tired, that sort of vibration is likely to start breaking the plates up. While it might be helpful to get that crud off the bottom of the cells, unless it is shorting something out, I'd leave it alone.
One bad battery will affect the others as the bank will only be as strong as the weakest battery. On top of that, if one is not putting out what it is rated at, one of the others miust compensate. This is why it is recommended that all batteries in a bank be the same age, capacity, and manufacturer. Getting them with a similar build date is just an added bonus.
I would take your battereis to a place with a carbon pile load tester. These will emulate a serious load such as that of a starter on a cold day or a power inverter starting a large device. The smaller testers are better than nothing, but often a battery is capable of outputing full voltage and might even maintain it's rated output long enough for them to call it a good battery whereas a carbon pile load tester is going to let the battery know you're really talking to it by putting a serious load on it. I know Batteries Plus will test the batteries for free for you if you have one nearby and they use the carbon pile load testers.
Your wiring looks really nice!
Someday maybe I'll take the time to do that someday.
I've never seen the blue anticorrosion spray, only the red, assuming that's what that stuff is on the terminals. I do see that you have several different kinds of batteries though (I can understand the price thing) and I think that might be part of your trouble. The easiest thing to do since those things are so heavy is to just drive the bus somewhere to have them tested. Be aware that you will have to break all the connections between the batteries for it to work accurately.
BTW....unless you really aren't planning on using the bus for some time, I would consider buying batteries now if you can. The price on lead has been skyrocketing like all metals and is only poised to go up so the sooner you buy batteries, they cheaper they will be atleast for the next 6-12 months I predict.