electrical drain

Yup. Stick a voltmeter across the battery when running. When warm and charged, the battery voltage should run about 13.8---14.2, in no case below 13.5 and not above 14.5. Check at a good fast idle ---to up to simulate low to med. cruise RPM

You still have the original style 69/ earlier regulator? Take a clip lead and hook the two connecting wires together, see if the voltage climbs up some with RPM increase. If so, bad regulator. If not, either bad field wiring or bad alternator.

If not voltage increase, move down to the alternator. Compare battery voltage to voltage at the alternator stud at a good fast idle. If the stud is very high, 15 or more volts, but the battery is low, much below 13.5, you have a wiring problem in the charge line/ bulkhead connector/ ammeter.

If battery and stud are both low, take your clip lead, unhook the field wire from the push--on terminal of the alternator, and clip the alternator field terminal to the output stud. Again raise RPM and see if either the output stud or battery starts to climb. If not, bad alternator. Might be as simple as worn/ stuck/ gummy brushes

If you actually think you have a drain, IE battery goes down sitting, remove the ground cable, and put a 12 v test lamp in series from ground to the battery neg terminal. If it lights or glows (examine in shade or garage low light) you have a drain.