Remove positive cable off of battery. Take a standard 12v test light. Hook gator clip to cable, then take light end and touch it back to positive post on battery. If test light lights up, up have a draw somewhere. Bare in mind, i have seen bad new batteries, starters ,and alternators.
Removing the hot wire from starter and alternator from each, one at a time, will eliminate whether they have shorted or not. Bad battery cables can also cause severe voltage drop. Especially when loaded.