Quick point I recently learned from a guy who knows his way around electronic engines: if you have an electronic DT466, ALWAYS use a minimum of two Group 31's, and if you have room, three Group 31's are worth the investment, regardless of the climate. Yes, the bus may start on one battery, but there are other factors in play.
His logic was interesting. The injector controller boosts the 12 volt supply to 120 volts to fire the injectors. During cranking, voltage falls into the 11.5 volt range or even lower. To compensate, the inverter section of the driver module has to draw more current. The lower the applied voltage to the driver, the harder it has to work to feed the injectors a steady diet. This causes degradation in the ECM drivers over time.
His observation was that busses with three batteries had a lower ECM failure rate by a fairly significant margin, and busses where there was only one battery had increased failures. His sample size was from a barn with something like 150 busses and 30 years of wrenching, so I'm inclined to believe his data.
For all you non-DT466e busses, I gots nuthin.
But I also stayed at a Holiday Inn Express, and the guy sitting next to me in the bar was a politician, so I could probably make-up something that sounds good. (It is an election year after all...)