Disconnect the battery (+) lead and install a DC ammeter in the circuit. DO NOT USE THE STARTER or you could overheat the light-gauge meter wires.
This will show you if there is any current flowing while the key is off. This will confirm any slow drain.
If there is a drain, start disconnecting things (headlight, taillight, blinkers, ignition coils, etc) until the drain stops. You've found it.
If there's NO current drain, then standard charging troubleshooting techniques apply - posted numerous threads elsewhere.
Good luck, and report back what/if you find.
Kirk