You have a short or an accessory still on. Get it tested at a shop or a GM facility. This requires some serious tools and a possible look at a TSB. Electrical problems are not easy to trouble shoot on a forum.
If you can't go to a shop? From Nitrous: "With the engine running look at your volt gauge. If it Is only charging at ~12 volts and the glow plug relay is supposed to be off. Look at the relay.
AAnother thing to try is to put a multimeter on the battery terminals and check voltage draw. Then, with the multimeter still on the terminals, lift off the fuse box cover in the engine bay and remove the glow plug relay (it will need to be removed with a 10mm socket IIRC). If the voltage starts to slowly go up or stay steady, you've found the culprit. If this is the case, you can leave the positive terminals on each battery unbolted while the truck is parked so there is no draw from the batteries."
The glow plug relay is the usual cause.