72 GMC 2500 - electrically dead!

[Bigblock50]

Truck is a 72 GMC Sierra Grande 2500 big block. Starts, runs and drives great, over 5 years. Now get this! This morning I decided to take the truck to the local post office 4 miles away. Starter cranked fast (fresh battery) as always, engine fired up, drove four miles and shut it off. Done in the PO, I started the truck and made a turn in the parking lot. Engine died ... but not only that! Everything electrical died, even the dome light. A few checks revealed nothing so I got it towed home. Three hours later ... no luck. Battery is strong, I checked and cleaned the terminals and the ground, tried hooking up a charger, nothing. There is no voltage at the fuse box or at any normally “live” terminals. No lights, horn, radio, etc. Continuity tester shows a good pathway between the battery negative post and the frame. Somehow no current is escaping the battery.

Ideas? I’ve about run out. 55 years of messing with cars and this is a first for me.

Bill in Luray Va