Battery draining but alternator and battery are brand new

I have a 99 gmc Yukon my battery keeps dying it all start one day i was driving and my battery gauge when down and killed on me while driving got a jump same problem happened changed the battery and the alternator still the same problem after the battery life dies took the alternator to get tested its perfect and brand new battery is brand new and dead as a door knob keeps happening whats my problem?