Battery has a slow drain.

I have a 2002 Yukon XL with 78,000 miles. I have been having problems with the battery going dead after truck sits for 4-5 days. Took vehicle to dealership and they said it was a bad cell in battery. I bought a new battery, drove the truck for about 3 weeks, let it sit for 4-5 days and the new battery completely died. I had to jump the vehicle. Turned key and it clicked and then started up. My battery display reads 15-16 amps when running. I had my stereo completely rewired and still something is draining my battery. The dealership blamed the aftermarket stereo. Not sure what the problem is. Can someone give me provide me with suggestions. According to the dealership, the charging system is okay. I am not sure if this makes a difference but, after the truck is jumped, the "TOW/HITCH" light is lit up.