Had this exact thing happen to me a few weeks ago. Drove from Astoria to Walla Walla, then up to Wenachee for Octoberfest in Leavenworth. About halfway to Wenachee, I notice that my battery gauge has stopped reading 13-ish volts and is now reading barely 8. Knowing how notoriously inaccurate the factory gauges are, I didn't really pay any attention, although the truck did seem maybe a
little bit more sluggish cranking over after the one snack stop we made...
Next morning I go to start the truck and I get maybe two cranks and it's dead. Walked to an Auto Zone and borrow their jump start box. Get the truck started and drive to the Auto Zone and pull the alternator to get it checked (thinking that maybe it had quit charging). Alternator was good. Pull the batteries to get them recharged inside and while I'm waiting I pull the regulator plug off the regulator (on the passenger side fender...little shiny flat box) and a couple of the plugs are corroded over with rust. Cleaned up the harness' plug and replaced the regulator with a new one ($16 and in stock at the Auto Zone), put the batteries back in, fire it up, and I got my 13 ish volts back; both on the gauge and on a multimeter across the batt terminals. Has been charging fine for the 1200 odd miles since.
So you might try the regulator...not advocating shotgun maintenance, but a regulator
is only $16
Ed
Edit: My 87's alternator is of course externally regulated. Not sure if or when Ford went to internally regulated alternators on these trucks...I'm sure one of the guru's would know...