Starting Batteries

It sounds stupid, but maybe you left the lights on. Was the battery totally flat? or did it at least click click the starting relay? That tells you if there is was a slow drain from something hard to find, like a radio etc. Headlights left on = flat battery, totally dead. Also, it is possible your dome light stayed on, or maybe if your rig has a battery boost function. That allows the house batteries to crank the rig if the start batteries are dead, and if the switch was left on, putting the starting batteries in the parallel with the house batteries, and allowing the slow drain of detectors, appliance controls, etc to bring all the batteries down. Were the house batteries dead too? Although you said it didn't have this boost function. If you drove a 1000 miles and all was well then the alternator seems good. Finally, check the torque of the starting battery cables under the hood make sure they are tight. Tell us more about this "killswitch." What exactly is that?