I heard a couple times in the previous posts about testing. I would quite guessing and replacing parts, and get a testlight or voltmeter and do some testing. If a voltmeter scares you, a testlight will work just as well.
I would get a helper, hook the testlight to a good ground and touch it to one of the battery + terminals. The testlight should glow bright. While holding it on the battery +, get some to try and crank the truck. If the testlight goes out, your batteries are dead. If it stays bright, then we need to move on down the line.
Move the probe of the testlight to the output terminal of the fender mounted solenoid. Put it on the decent sized wire that runs down to the starter. You should have no light. Get someone to try and crank it. The solenoid should click, and the light should light up.
If the light did light up, crawl under the truck and put the testlight on a good ground and touch the large lead from the battery. The testlight should light. Then get someone to crank the truck. The light should stay lit. If it doesn't, that cable to the starter may be bad. If it stays lit, move your probe over to the smaller wire that comes from the solenoid on the fender. The light should be off. Get someone to crank the truck. The light should light up. If it doesn't, there is something wrong with that wire from the solenoid. If it does light, then there is something wrong with the starter.