Your restatement of the problem is correct. I just checked the battery voltage without the engine running and it is 12.56volts.
I will try the extended drive at higher RPM test you mention and see if the problem is reproducible. Most of my driving is stop and go in town and under these conditions the symptoms are the same no matter how short the "engine off" time is.
1990 Mercedes 300SE (147,000)
1985 Mercedes 300SD (203,000)
1983 Mercedes 240D (167,000)