I had this same kind of problem last year due to the grounds for the block. I got the truck with large speaker wire as all the battery cables and set screw-type connectors (set screw for the cable end, usually nickel or gold plated at the parts stores, highly recommend). Both batteries are now grounded to the frame and from those same bolts are 2 more cables grounding to the block. There is supposed to be one on each side of the block, but I was missing one bolt and couldn't find a replacement easily, so they are both grounded to the passenger side ground bolt. I also ran an 8 gauge wire from the CDR bolt on the back of the intake to the wiper motor and another from that to the driver's side hood hinge bolt. You can never have enough grounds.