Have you checked the current output of the alternator? One bad diode will drop the current capacity of the stator to less than one third of its rated capacity, without affecting the alternator's ability to produce normal voltage. I've seen "55 amp" alternators with an open circuited diode in the rectifier bridge produce less than 15 amps max- - - -runs the vehicle OK in the daytime, but slowly discharges the battery with headlights or a heater fan running. Have someone who knows how to use an ammeter check the alternator for max output.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!