Nice black truck!
If you were good at photoshop, you could make that picture look like you was driftin'...
Your problem is starting to sound like a bad battery.
It should measure 12.6 if it's fully charged (engine off, key turned back on before UG shuts off, look at voltage). Or just measure it with a DMM after it's been off for a few hours. 12.6 is fully charged typically. The UG trick may not read correctly as it will show the battery voltage before it settles.
Your voltage reading sounds good for a happy alternator. Maybe a bit high? A bad battery or connection could cause it to read high possibly. I'd start mine, but it's inside.

I've replaced batteries that were one year old because they wouldn't crank. A fresh battery has kept me from hours of wasted troubleshooting a few times. This seems to be more common when the battery is infrequently used, rather than every day use.
Make up a cig lighter to DMM cable so you can measure your voltage easily from inside. Be careful you don't short it out when it's plugged into the lighter socket. That will be a fairly decent measurement of your actual battery voltage.
I just replaced the 4 year old battery in my wife's car. We've been seeing minus temps here, and it would barely crank. The new battery spins the engine well now, even at sub zero. She's a happy

. I'm happy too.
I'm a little worried about my battery in my 80... It's a cheap battery I bought to get it running two and a half years ago. I don't trust it, and I try not to put myself in a situation where it's a possible failure.
You can almost bet the battery will test bad because they want to sell you a new battery... Sometimes they're right.
Keep us posted. You might save me some time some day in the future with what you find...