charging system blues

You may have a bad battery. First thing to do is charge the battery and have it tested.......with a load tester

Only problem with TB's suggestions above is that there might be a wiring problem causing "the problem."

I tend to divide these problems into "areas.........."

The field wiring
The output / charge side wiring
The components, alternator and regulator

To do the test above and eliminate the field wiring as a problem, get some alligator clip leads which you "should have" anyhow. Remove both field wires, ground one, of the alternator field terminals, and run the other to the battery

Now monitor battery and see if voltage "comes up" with throttle increase. With the engine running "fast" (simulate) low / medium cruise RPM, voltage might be over 16V. Turn on headlights, blower, etc, and see if it "keeps up."

Also compare the battery reading to the alternator output stud. Easiest way to do that is hook one lead of your multimeter to battery POS, and the other to the alternator output stud. Bring up RPM to what you judge to be no more than 16V in the first test, "a good fast idle" or little more. You should not read more than one volt, maybe 2 at the most, the less the better

Voltage regulator MUST be grounded. Remove it, scrape around rear of bolt holes, firewall and regulator, and remount using star lock washers. "Work" the VR connector in/ out several times to scrub it clean and also visually inspect it for loosness and corrosion