What causes a slow cranking on a truck?

I had the same problem. I had been using a very poor battery that was almost flat all of the time. All battery connections turned out to be in good order, worried about the engine begining to seize up hbut found it turned freely by hand. the problem turned out to be the starter motor that had broken down due to constant use of a poor battery condition. Please check your battery condition, connections, that engine turns freely and your starter motor. A lot of garages do a start and charge check very cheaply.