Here is my story. It was a 2001 with a Duramax, so 2 batteries.
Came to find the truck dead. Asked my wife what she did as she was the last to drive it. I was joking, but of course should have kept my mouth shut. Explain I'm joking and there is nothing she could have done to cause it.
Charged both batteries and it was good for a couple of days, and then dead again. Part of the problem is I have a short commute, so batteries never get a good charge, but they should last more than 2 days.
Weekend comes. I've stuck the truck on the charger so I have a good charge. Disconnect on batter entirely so I'm only fussing with one. Disconnect the negative and put an ammeter in line to see what's happening.
I see a draw of about 2 amps for maybe 30 seconds, then a slight draw for a minute or so. Repeat. Repeat.
I start pulling fuses one at a time. Of course I have to wait a minute or so as it's not a constant draw. One at a time. Cursing all the way. Well, I finally pull one and don't see the draw. Forget which one it is now, but it controlled stuff in the cab. Look online to see what was controlled by the fuse and somehow glean onto one item being the circuit breaker for the electric motors for the seat. Sure enough, I check the control and the seat cover has the control to push the seat forward jammed full back. Move the seat cover, replace the fuse and the mysterious draw is gone.
Put everything back together and go inside.
Remember above where I told my wife there was nothing she could do to cause it? Well, yeah dumb me (still kinda in a pissy mood) tells her I was wrong and it was something she did. God, some people never learn.
Anyway, if you do what the above post suggests, then know that circuit breakers can draw and then not draw. So be sure to check for more than a minute.