Do you have a voltmeter or testlight? If you do, put it on the large lug on the starter relay on the fender, the one that goes to the battery. It will light up or show voltage. WHILE STILL HOLDING THE METER OR TESTLIGHT ON THE LARGE STUD, get a helper to try to crank the truck. If the voltage drops to zero or the light goes out, your batteries are dead or there is a poor connection at one of the batteries.
You can move your meter or testlight around to different spots to narrow down the problem. You must check for voltage WHILE A HELPER TRIES TO CRANK THE TRUCK. Voltage readings just sitting there tell you nothing.