Charging Issue, Stuck

Hello, and thanks for any help in advance. 2010 tundra 5.7L platinum 207k miles.

We had a cold snap last week and the truck started up kind of weak. The battery was 5yrs old so I assumed it was time for a new one. Put a new one in and the truck started up then died. From reading on the forum I have learned that that is normal. Fired it up again and it sounded weak ( low idle). Had the O'Reillys guy test the new battery and he said the new battery was good but that I had a bad alternator. (No issues with the truck until the battery was replaced)

The truck had the original denso alternator and I thought it was time for a new one anyway based on the mileage. I ordered a new (not reman) denso alternator using my VIN from the dealership and installed it. Same problem. Truck fires right up, battery shows 14.1V on the multimeter. Idle sounds normal for 10-15 seconds then sounds weak again and the battery starts to drop voltage into mid 12vs before I shut it off.

I have checked all the fuses. All fuses look normal, not blown. All fuses related to the ALT have been replaced (ALT-S 5, AM1 7.5, LH-IG 7.5) . No lights on the dash and no other noticeable electrical problems or problems in general. Only work done on the vehicle recently was the radiator replacement 4 months ago.

No aftermarket mods that I can find or know of other than a trailer brake controller.

I had the new battery tested again and it's good. Could it be the new denso alternator even though the battery shows 14v on start up? Any thoughts on where to go next are greatly appreciated. Thanks.