Found out today why my alternator belts were chronically loose, no mater how often I would adjust it.
New alternator required, due to vibration elongating the pivot hole and wearing away the pivot bolt. The new alternator also fixed my low voltage problem that I had convinced myself was normal. I was charging at 12.8-13.2 before the alternator replacement. Now I've got a consistent 14.2, more along the lines of what I'd expect. Maybe now my headlights will look less like flashlight beams.
310,000 miles and 14 years on the alternator. It was probably time.