I reckon I’d first try tracing the cables to see where they go, but it sounds like you may not be able to tell which Main Switch (“cutoff”) is which? On ours the chassis switch is rearmost on the panel, intimating the engine’s position on the coach, but that might not be an assured design on any one coach configuration. And tracing wires with that many batteries involved may be an intimidating task.
An option would be to first turn off both cutoff switches, then disconnect the ground cable(s) coming into one of the two battery types; I’d guess the 1131’s would be handiest since they are all together on one shelf and you should be able to see one good sized cable entering that shelf and attached to the negative post of one of the three. Commonly, the positive cables will have red tape or coloring on their ends, and negative cable ends are just unmarked/untaped black. With one (or more) black umbilical removed between battery set and coach and fastened securely well away from any battery post, turn the cutoffs back on. Try starting the coach. Obviously if it starts, and you know for certain that the threesome have no large ground wires coming into them, then Steve is correct and the 4 AGM’s must then be chassis set. They do, after all, rate a good deal of cranking amps. Approach this task with caution; if you don’t feel comfortable handling battery cables that can spark, then don’t try it. Ask a certified electrical tech.
Without actually seeing the setup, I’m shooting in the dark here. But any of your mechanics could’ve done the same thing, likely much easier than you as a relatively inexperienced coach battery inquirer. If you approach the cable disconnect experiment, please use care that a wrench or other tool used on the negative cable(s) doesn’t inadvertently at the same time hit a positive post or bare cable. You don’t need sparks in your face.
Joel
