Josh,
Well Sir, first off, I'm either numb or, just not educated on what "MS and "DC" mean in your situation. And, you say all three are wired "together"? Your comments state you think this set is for starting the coach? Well, maybe it's just me 'cause I too don't understand this scenario.
If, for some seriously odd reason, they are all tied together for starting, then where's the house batteries? If one of them is for starting and, the other two are for house battery operation then, how could they all be wired together? Please don't get me wrong here. I'm not chastising you, or in any way, degrading your situation. I'm just trying to understand what you've got in front of you.
Scott
