My 2000 GMC Sierra 1500 5.3L lost ALL power. No electrical power period. This was after driving in rainstorm. Truck drove normally and I shut it off at destination. Went to restart and it was dead. Battery is charged. Out of the blue it started once but would not run. Then no power again. Intermittently power would come back but when tried to start I heard a click and no power again. Cleaned all battery connections. Checked all fuses and fusible link. Very warm and sunny (ie. dry) the next day. When I hooked up battery, power is back and vehicle appears to be running normally. Alternator is putting out 14V. Removed the starter and cleaned all the connections and still is working fine. Starter connections did not appear corroded. Is this the starter getting wet or on it's way out and grounding out the electrical system?

 

sounds like a electrical problem to me.

 

Make sure your battery is actually fully charged when you're having issues with it starting. When the truck is running, you should be getting 14-15 volts coming out of the alternator to charge the battery. You might consider having your battery/alternator checked at a parts store for free.