83 K20 charging problems

[Smitty1962]

I recently developed a charging issue with my '83 K20 w/ 6.2L diesel that has me stumped. It suddenly stopped charging (voltmeter on battery reads 12V with truck running and volt guage in dash also reads low). I tried the obvious and went and bought a new alternator (regulator is internal to alt) and still no luck. If I let it idle it does seem like the voltage will very slowly go up to around 12.8 or so, but run it down the road and voltage drops again. Here is what I've tried / know:

* Batteries seem to test OK with my cheap $40 100A load tester (they test to 800 CCA, which is what the batteries are).
* With the key on, I have 12V to the wire on alt terminal 2 (red wire) and about 10.8V on the exciter lead (term #1). I'm assuming the lower voltage on the brown wire is because I understand there is some resistance provided by either the key switch or volt meter in the dash which is lowering the voltage.I can't get to the leads to measure when it's actually running without removing other stuff.
* With the truck running, the voltage on the output terminal is about .2v higher than what I read at the battery but still far less than the 13.5-14v it should be (again, seems like it's charging very slowly).
* I've cleaned all of the battery terminals and removed the ground cables from the engine, cleaned, and re-tightened.

This thing has me baffled, anyone have any ideas??