The first thing I would do is get the truck started and check the voltage. You should have at least 13.2 to 15 volts if the charging system is working correctly. With the engine running and the high beams turned on, you should not have less than about 12.8 to 13 volts. If it's less than that and you're certain the alternator is good, then I would suspect something putting an excessive load on the system or an undersized alternator.
But don't forget the simple stuff. Is the fan belt slipping? Is it too loose? Are the battery terminals clean and tight? Do you have a good clean connection to ground?
Get ahold of a good ammeter and measure the current with the system turned off. If there's a current draw with the ignition off, something is wrong (unless you have a modern vehicle - then you might have minimal current drain from electronic components). Start disconnecting things, one at a time, until the current draw stops. Reconnect everything except the last thing you disconnected, and see if there's any current draw. Doing this, you can isolate the cause of the draw. Then you have to figure out why the part you've isolated is drawing current and what to do to fix it (or replace it.)
If there isn't any current draw with the ignition off, turn the ignition on and measure the current again. You should have minimal current draw.
Finally, start the truck and measure the current again. If your alternator is rated at 40 amps and the system is pulling more than 40 amps, you either need a bigger alternator or you need to find out what's using all that current and fix it.
IOW, it's time to start troubleshooting.
