Just went through a similar issue. The one wire alternator didn't excite until 2500 alternator rpm. With the combination of a large alternator pulley and small crank pulley, there wasn't a lot of time that the alternator was putting anything back into the battery. To make things worse, the alternator had a bad diode and rectifier so it was only putting out 15 amps when it was charging.
Had it rebuilt and now it works fine.
Changed the charging system to a larger 74 amp constant output alternator with a 6" crank pulley. The excite power wire runs through a heavy duty toggle switch on the dash so the alternator can be turned off during a run. -Al