It turns out that the main positive connection (forward battery, inboard side) had pushed up against the main ground cable (aft battery, inboard side). They cross just behind the positive terminal as the ground traces forward to the main switch. The braided ground had just enough wear on the insulation that it shorted out against a protruding "shoulder" of a positive connector.
I quickly pulled the battery tray forward, releasing the short. The flat braided ground was burned completely apart about 1/4- 1/3 of the way across it's width in that instant.
A new NAPA round ground cable was purchased, and all the wires were removed from the box, the box removed from the truck and inspected. The original rubber gaskets (the rubber rings that seat on the hole's metal edge) that protect the wires passing through the holes into the box, had disappeared to rot. I had to use a step drill bit to open up the passage holes enough to get the new round cable in, and put in new rubber gaskets. All wires were checked and the old electrical tape that had been in there (a long time) was changed and new insulation (corrugated plastic and a section of rubber garden hose) was placed around the wires and connections to the batteries. It gave me an excuse to wire brush all major connections (lots of crud built up on the main switch terminals). Now it's all new from main switch to battery with lots of insulation on the positive terminal.
I wanted to relate this, as I can see the proximity of the connections on the negative and the positive terminals of the batteries is compressed together against the rear wall of the box, when the batteries are slid all the way back (at least with my large sized batteries). A lot of posts speak of adding extra electrical wires (inverters, lights, heaters etc...). In my case this extra bulk caused pinching between the burdened positive pole and a fraying ground cable.
I was lucky that it happened while I was closing the battery box door, and not when bouncing on a trail. That's also the last time I store my extra quarts of oil next to the rear battery! Not smart
.Wonder if any one else has noticed problems with this electrical "short" coming
?Ed
