Slow crank/ No start
 

2004 6.0 ran fine parked overnight now cranks slow and will not start

The motor has to be spun at a decent RPM for it to start, if its too slow it won't fire. One of the things that happen often is the AC compressor/or clutch start to sieze up causing a slow crank. Loosen the belt and then try to spin each idler pulley,ac pulley, water pump, and see if any are tight or fell grindy. If all those are good then might hav a battery that is dieing on you.

I loosened the belt and all the pulleys seem fine. I charged batteries and they are both at 12.7. Still turns over very slow.

Have your batteries load tested. Just b/c they have a good charge doesnt mean they have sufficient amps to get the job done. How old are they? Also check and clean all battery contacts and all grounds.

batteries are not even a year old. Cleaned connections and terminals. Could starter be going bad? Also night before it hesitated but then ran fine. Next morning this is what happened

Yep. Thats a real possibility. Sometimes you get a bit of a warning before they go out, and sometimes you dont. Also, I forgot to mention, make sure your cables are good.

Put new starter in and now cranks. Tries to start but will not run

Does it crank over really fast, or slow. If slow, then your batteries are likely the culprit. It must spin rather fast to start.

I plugged truck in for a few hours even though it was 60 out. Truck started and now has engine light on and was running rough.

Probably just from you turning it over so much. Run it for a day and see if it goes away