Battery drain

You would need to do a battery drain test. This procedure can be a bit tricky since you can blow out your fuse on your multi meter (I have a few fuses on hand cause it happens to me). If you have a digital multi meter and an alligator clips for the leads I can try to guide you, but I would recommend taking it to a shop for proper diagnostic.

What I have seen at times is the blower motor resistor burning up causing the blower to run all the time even with the key out. If you do notice the blower running all the time you will need to replace the resistor and connector. GM actually had a campaign for this issue.

If you have any aftermarket stuff installed like alarm, radio, DVD players, etc. that can be an issue also. I will post a link for the draw test but I would recommend a multi meter to actually measure the amperage draw since the electronics on these vehicles are more sophisticated.

https://www.2carpros.com/articles/car-battery-dead-overnight

Hope you find the link useful.