2002 GMC Savana Battery keeps Dying

I have owned this 2002 Savanna for the last almost 2 years. Recently, I went out and the battery was dead. We jump started it and over the course of a week, the battery kept dying overnight. The rear brake lights had also remained on while the vehicle was off w/ no keys in the ignition, but I put the keys back in, and turned it forward and back, and they shut off. I went and removed the battery that was dying, and bought a brand new one. After that, I checked to see that all possible wires were covered and not exposed anywhere. The new battery has consistently died (usually overnight). We had the alternator tested, and it seems to be functioning correctly, also, the volt meter in the van reads 14 everytime, even though it is reading fine. We just can't seem to figure out what is draining the battery overnight?