No power Had my battery checked?

Recently bought a 2013 gmc acadia. Drove for 2 weeks with no issues. Then, the battery gadget went all the way up for a few minutes then returned to middle of gauge like normal. The next day, the gadget was in the middle then started going low. Then the radio shut off and had no power. My buddy came and got it started with his jump box. Had to pull over a few times because it wouldn't stay on. As soon as I pulled up to my house, it died again. I put the car in park but couldn't get the key out. I replaced the alternator thinking this was the problem. A week later, still no power. Had my battery checked and auto zone says it's good. I have checked fuses. I found an inline fuse coming from the alternator. It says 200 on it but can't see how to change it out. Please help. Idk what to do