Several things could cause it, but the first thing you need to do is check the fusible links. It sounds like you blew the one between the battery and the distribution block on the firewall.
First, make sure the front battery has 12V
Then, check the small wire coming off of the dogbone (battery interconnection cable) that runs along the inner fender and across the firewall to the diamond shaped two post distribution block near the master cylinder on the firewall.
If the wire is ok, check the distribution block with a voltmeter to see if you have 12V at the block. This is the main 12V power for the truck.
If you don't have 12V there, the fusible link in that feed wire is blown. I would get a fuse holder and put a 30A fuse in it. Make a jumper wire from the battery to the 12V junction block with that fuse holder. Turn off the heater fan (if it's on) and all the lights then connect the jumper to the block. If it blows, there is a short on the truck side of the block that you need to find. If it is ok, check the function of each accessory individually. If they all test out, try to start the truck.
If it all checks out, replace your jumper with a new (properly rated) fusible link on the feed wire and you should be good to go. Sometimes the fusible links fry from old age without a major problem. I would not recommend just jumping the fuse block with an unfused circuit because if it were to have a short, there could be a fire or you could damage the truck's main wiring harness.
Later,
Joe
