What could be pulling down my battery over night other than the obvious things? on 2000 Mitsubishi Montero Sport

Have a second new alternator on now and if I leave the cables on over night the battery is down. No lights, etc. on that we know of.

there are several things that can drain a battery:
if the battery is more than a couple of years old and has been dead multiple times, it may be sulfated to a point that it will not charge, it is best to have a battery charged off the vehicle and tested.
also any number of computers or modules on a vehicle can "stay awake" after the key is turned off. most are supposed to shut off after a few minutes. these are things you can't see, a seat module for example.
a parasitic draw test can be performed to check for these kinds of problems
1 more answer
It is most likely your altenator