cranks slow over first cylinder

[Waterloo]

Almost sounds like a parasitic draw if the voltage drops that quick. Which will impeded the starter... How old are the batteries? And have you ever crawled under the truck to visually inspect the cabling running from the battery box to the motor and accessories? I am sure you read my experience with my factory wiring, exposed cable, no jacket?

I would also check the ground cables from the starter to the frame rail, and the one from the cab to the frame rail, just opposite the starter one, Rawze caught that one. Hell, I never knew it was there! And it was shot. That is where I would start, as it is fast and free... If all looks good, go on to more extensive diagnoses.