Reads like you have a problem
New battery should be 12.8 volts to 13v, at rest
5 year old battery 12.3v, near end of life
12.2v or lower is end of life, and cold morning, "click, click, click"
Since your new battery was drained, you can cut it some slack, but it should come back up to 12.8v after charging.
After startup battery voltage should show 14.5v or higher, under 15v
Alternator's voltage regulator puts out "recharging" voltage, 2+ volts, until battery is recharged, that could last 5 to 15 minutes, depending on battery's condition.
Then voltage regulator will switch to "maintenance" charge, 1 volt, to keep battery charged without "cooking it".
Once started the alternator provides all the power for the vehicle, recharging or maintaining the battery is a secondary purpose, battery can actually be removed from a running vehicle, it is no longer required by the electric system.
Battery's only purpose is to start the engine.
I would retest battery voltage
After is sits for an hour or longer, thats "at rest" voltage
Start engine, you should see 2+ volts more at battery, 14.5 to 14.9v
Increase RPMs to approx. 2,000 and hold it there, volts should go up and then drop back down to 14.5-14.9v, that's the voltage regulator working.
If startup voltage is lower, 13.5v, and then goes up to 14+volts with RPM increase then alternator has an internal problem
After driving for 10 minutes or so, don't shut off the engine and test battery voltage again.
Should be 1 volt above "at rest", so 13.5-13.8v, turn on all the lights and heater fan to high.
Voltage should drop and then come back up to 13.5-13.8v
If voltage drops and doesn't come back up then you have a failed field in the alternator.
Test alternator wiring
Key OFF
test B+, wire on the large stud/nut on alternator(you don't have to disconnect wire from alternator), should show battery voltage, exactly the same, if even slightly different then that wire has corroded connection at one end.
Remove 3 wire connector on alternator.
test Yellow wire, should show battery voltage, if not then blown fuse or fusible link
test Green wire, should be 0 volts
turn key ON, engine OFF
Green wire should now show battery voltage, that is your Battery Light wire, it is also the ON/OFF switch for alternator, so an important wire.
It runs through ignition switch and dash so can show slightly lower voltage, but .1 to .2v. not alot
In 1989 battery's Positive and alternator are connected at Starter Relay on the inner fender, one of it's larger studs is the power distribution point of the truck, so all power for the truck runs through that one connection, make sure ALL the wires connected there are clean, use wire brush or emory cloth to remove and corrosion on any wire ends.
Battery's Ground is equally important, make sure its connection to engine is clean(bare metal) and tight
DC voltage is like a ONE LANE circular highway, if everyone is driving at 80mph then every one can go 80mph, if there is road construction(corroded connector) which slows traffic down to 20mph then EVERY ONE has to slow down to 20mph
"Keep the highway open", clean those Grounds, lol.