Mike and Fishin, here's what I've found after watching things for a couple of days:
The chassis battery didn't so much stabilize since I gradually watched it go down to 12.58V; then on the following morning I found it back up to 12.75V. So the BIM must have opened during the night - perhaps because it was cooler?
Then today, I found the chassis battery was down again to 12.57V. And I noticed the Magnum was on Float at 13.0V.
The other day when I first noticed this, I had returned from a trip to Phoenix where the temperature was 115F that day, and the AI stayed hot all the way home and probably didn't cool down in my garage for 2-3 days.
So Mike, I agree with you that the Magnum must be lowering the Float voltage to compensate for the temperature.