03 Cummins not wanting to start/ long cranking
 

so i need a little some help have 03 with 130,000 miles ish on stock this been ongoing problem the truck started with not starting one day and after killing the battery's twice it started so it end going to dealership tested could find anything couldn't fine anything took the truck from nh to ohio never had problem only shut off at night and plugged it in it was fine one the block heater,the next morning after getting home left it unplugged about 10 deg. in the am barely started, so thought two new battery's at cause even cause the factory ones tested good that were 12 years old so that helped some so spins faster now still long crank time before it starts any idea's truck has hadn't real good life when comes to service (oil change filters ect) cause it work truck and not living thing it doesn't need to be serviced it's fine <----- not me saying this the boss of the farm. so any help would be good so cause it's going to back ohio in april and i'd like to get the fixed/sloved, i kinda leaning towards injectors ?? have already done #5 i think but if have to do all of them maybe upgrade to a few more hp would be plus !