Okay... I have it set up in the cabin. Cruising at 40-50 mph with AC off = 13.25v steady.
Once I turned AC on, it slowly went down to 13.10 or so. Then in the mid 12s once I stopped at a light.

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1987 300TDT - 195,000 (Original #14 head)
1993 190E 2.3 - 105,000
1981 300D - 250,000