Charge up the battery, get the truck started, and measure voltage at the battery with the engine running but everything else electric turned off. It should be in the high 13s to low 14 volt range. I don't think a 1994 would have a smart charging system. If the voltage stays in the 12s, the charging system is not working and the battery is going to eventually run down.
If that is OK, alternator is working but something may be draining the battery while it is off. The test for this is to disconnect one terminal from the battery then connect an amp meter between the truck and the battery. If it's a heavy draw you could use a 1156 bulb. The bulb should not light. If there is current draw, start pulling fuses and disconnecting stuff. The alternator would indeed be suspect. The alternator's big output wire is connected directly to the battery, so it is live when the key is off.