If you have installed a new alternator and the system voltage still fluctuates at the voltmeter, you may not have needed to buy it.
Assuming that you did not receive a "bad out of the box" replacement unit, the problem most likely is in the wiring or the gauge itself. As info, the voltage regulator is integral to the alternator - you get a new one when you buy an alternator.
Another indication that the alternator was functioning properly is that the system was still fully charged.
I would let things be for awhile and see if there is any sign that the battery is actually being drained.
If the gauge continues to fluctuate, but the truck continues to run for weeks, I would look for an intermitent ground on an unfused hot wire.
If you drive a lot at night (and run the wipers and heat/AC bloweer) you'll know real quick whether or not the battery is not being charged. It a day or two it will be dead and unable to turn the starter motor fast enough to start the engine.
If, on the other hand, the electrical load conditions are light (short easy engine starts, no headlights, no wipers, no heater blower etc) the battery will provide power for quite awhile.
It sometimes takes 7 to 10 days to drain the battery down to the point where it cannot spin the starter fast enough or fire the ignition system (under 10 volts).
In either case, while the battery is losing its charge, you will notice a slow steady drop in voltage at the dash meter. Without the alternator working, the voltage will drop suddenly when the load increases (e.g. turning the headlights on). This happens even with a good alternator - but it doesn't drop as drastically and bounces back quickly. If you turn the headlights on with a dead alternator, the dash meter goes way down and very slowly climbs back up. Each time it drops, it comes very slowly back up - and it never comes all the way back to where it was.
An alternator that has stopped charging (usually due to failed diode/rectifier in the voltage regulator) shows itself as a gradually lower voltage at the meter. I think the regulator is set to maintain an output voltage of about 13.5 volts. This normally is enough to keep the 12.6 volt battery fully charged.
When the alternator output ceases, the entire electrical load is drawn from the reserve power in the battery.
