If Battery keeps draining overnight and it's not the alternator or battery?

If you are certain it is not a faulty battery or alternator problem, the next step is to find what is shorting out or drawing current with the ignition in the off position. Could be a number of things: Have you had aftermarket accessories installed (shift light/digital gauges/stereo/amplifier/car alarm/etc?) that would be where id start first, then use a simple circuit tester.

Disconnect the battery, clip a lead on the + battery wire, any circuit that is drawing current will make the tester light up when you short the circuit/wire under the dash using the probe..this will only show you what can pull current when the ignition is off

Hope this makes sense