My truck won't start even though I changed the battery and the alternator. It won't even crank

check and see how many amps/watts your alternator is sending. Could be a bad alternator even if it's new. (We just had to do this on a car..it was sending the right watts but not the rights amps, even though it was new. It was warrantied..switched it out and worked like a charm)

I doubt he can check the alternator if it won't crank, honestly replacing the alternator sounds like you wasted money unless it was done for another reason. Is power getting anywhere else? Like lights? Radio? Can You hear the starter solenoid click when the key is turned?