I've had this problem, last 6 months. Battery will be dead if I don't drive the truck for 3-4 days, which is frequent since the truck has 315k miles on it and is a weekend warrior/back up vehicle.

I put my multimeter inline on the negative side. My drain is 1A DC. I know the usually acceptable drain is around .2A. I pulled every fuse in both the in cab box and the underhood box and nothing made a difference. 1.08A for everything. Thought I was close a couple of times until I realized the key was in the ignition and radio still running (volume off). Don't you hate false positives? After that, everything was steady except for pulling the radio fuse (which dropped it from 1.08 to 1.07A).

Now, the battery will seem to hold a charge just fine, but just slowly drains. Once it's charged, it will drive fine with no problems. And if I drive it every few days it is fine.