Old Skool Battery / Alternator question

[Crazy Dazz]

You'd think, after all these years, I'd be able to nut this out...

Old Skool set up in a Nissan Patrol diesel, so no sophisticated electronics.

Battery is flattish after sitting for around 10 days. There's power, but not enough to crank.
Jump start with difficulty, and take it for a long drive.
Following morning, dead again. Roll start and take even longer drive.
Few hours later, dead again, Roll start and long drive, during which speedo and gauges stop working.

Today:
Battery reads 10.6V. Power but no crank. Jump starts easily.
At modest revs, alternator putting out 14.6V.
At idle, alternator putting out 13.6V

Usual Suspects:
1) Battery is stuffed. (It's "sealed" type, so no checking fluids.)
2) Alternator not charging (perhaps intermittent??)
3) Some weird power drain (but there's no alarm, minimal electronics, etc.)