More multimeter tests to be done:
With bike running, measure voltage at idle across the battery terminals and see if it slowly drops over time. It shouldn't. Then keep the motor revved, and measure again. It should read above 13 volts when revved.
If either of the above tests fail, it is likely your stator, voltage regulator, both, or a parasitic draw as the poster above mentions.
There are more multimeter tests to determine whether it's the stator or the voltage regulator, which you can find in the service/shop manual (free, just search for it).