Hey - I have this same issue on my 2003 GMC Yukon 2wd 5.3L with 180k miles. My battery was dying over night pretty much every night. I pulled the IPC/DIC fuse and it has sat for more than 2 days and battery is still good.
I've read about and watched a YouTube video (South Main Auto) where the mechanic has a 2003 Silverado with this same issue, he traces the parasitic battery drain to the gauge cluster, and he sends the cluster away to United Radio and they refurbish it and the truck is good to go - it fixes the drain problem. They charge $175 for this service (up to $350, depends on what the tech finds wrong). One of the issues cited in the South Main Auto video (he calls and speaks to the tech to worked on his cluster) is that when they built these clusters in Mexico they used no lead solder, and that causes a "silver migration" issue -- basically sounds like it leads to some corrosion on the gauge cluster computer board and that can cause the constant power draw.
I'm just wondering if those of you who had this problem had your cluster serviced or replaced the cluster with a new one, and did it fix the problem? Can I reasonably rely on my issue being the gauge cluster, or could it be OnStar that's causing my problem somehow? How can I permanently disable OnStar? I don't have a need for that anyway since cell phones are now a thing.