Larry, and all,
Thank you for the advice.
I just replaced my voltage regulator.

Did it all by myself.
It made me feel good to actually fix something on my own car.
Pulled the TE up on ramps, and went in from below.

It took a bit of manipulation to get the thing in there, and some more $#@$@# words of encouragement to get the screws in, but I now have close to 14 VDC when driving.

I have resolved to now keep a DC voltmeter plugged into each of my Mercedes' lighters and actually watch the thing to monitor charging system/battery performance.

The TE has a genuine MB battery, but the SD has a $60 National Auto Parts / Exide battery. So far so good with both.

The part cost under $18 from PartsShop (thanks Phil!) and took care of the problem.
I can now motor happily, and hope this one lasts another 122K miles....