Just a couple tips...

You will always have a slight drain on the battery for the clock and aftermarket stereo stacks. This is to maintain the internal clocks.

Make sure you're using a fused volt-meter(might have to pull the back panel off to see) and check the voltage output of the battery at 100% charge. Then like said before, check alt output cold and hot. I would suggest revving the engine to your highway driving rpm as well to see if there is a problem there. soo, four checks:
Cold
idle
at rpm
Hot
idle
at RPM

Since headlights create the largest draws on power, do the same checks with headlights on and off. If the issue arises with the headlights on, inspect the circuit(fully) before considering the info below.

If all the voltages are GREATER than the battery 100% voltage (usually 12.5 - 13.1v for just batt, running alts are generally 13.5-14.6v) then your alternator is not the issue. If there is LOW VOLTAGE at idle, it is likely a diode has burnt in the alternator(Replace the entire alt. It is possible to rebuild though for confident individuals.). Low voltage at RPM is a breakdown in the winding insulation(alt internals which is just easier and cheaper to replace the whole thing).

My wiring diagram(1986) shows that the alternator houses its own regulator but if for some reason yours does not then any above failures could be living in the regulator.