One of the popular questions is "Which is better 12 volt in parallel or six in series.
Well there are no six volt batteries in RV's. only 12 volt, SOME of those 12 volt however are split in two for easier hangling (Each half is of course six volt)
Here is what happened.
The Bomb had a cell or two short out.. So now instead of being 12 volt it is perhaps 8 volt, or six volt (Depending on how many cells shorted) the others thus tried to "Charge" it, seriously overcharging and BOOM.
By they way... This is a very rare failure, and it can happen with six volt packs as well if your converter can push enough or if you have multiple pairs.