Are the connections at the battery tight? Any corrosion on the battery terminals? How's the ground cable from the battery to the chassis? And the ground strap across the motor mount?Need to tap the hive mind for a weird problem. 98 c1500 ext cab, 5.0l battery is 6 months old and tested good with an electronic tester today. Get in the truck, key to On, dash lights up normally. Headlights work even. Turn the key to start .. all power vanishes. Key to off ... still no headlights, no power in any key position. Come back a couple of days later ... power's back, headlights work ... key to start ... power takes a holiday again.
Sounds to me like the power from the battery isn't getting into the rest of the truck. A relative trickle makes it through to run lights, but the big draw from the starter overloads the weak spot in the circuit.