I went to start my 2000 F350 with 287k miles this morning after it had been sitting for a couple days. I may have gotten 1 revolution before I heard a thunk and then nothing. The starter engages but wont turn over the motor. I thought the batteries might be dead so I put a charger on them and they are fully charged. I can feel the starter trying to turn the motor but it just can't. Any suggestions on where to start looking?

 

I would bet the batteries fail under load. Take them to your local auto parts store and they can load test them for you.