Do you have a voltmeter? If you do, get a helper to turn the key. You must do voltage measurements while someone is holding the key to start.
First thing to do is put the meter leads on the battery posts, hold them there, you will probably have something around 12v. Keep the leads on there while the helper tries to start it. If the voltage holds, good. If not, you need a new battery or it needs to be charged.
If the voltage held, your helper can let off the key. Keep one lead on the battery negative, but move the positive meter lead to the starter solenoid on the fender. Put it on the large lead that comes from the battery. While holding both leads, get the helper to spring the truck over to start. If the voltage drops, your + battery lead is bad, or you have a bad connection at the battery terminal.
If the voltage held, then keep the + lead where it's at, and move the lead from the negative of the battery to a good clean metal place on the engine. Hold both leads and get your helper to turn the key to start. If the voltage drops, then your negative battery cable is bad or the negative connection on the battery is bad.