Truck Repair: 2006 dodge diesel, dodge diesel, highway speed

Question
i use the truck for work and when it is hot out i don't run the motor. i have a constant draw from the batteries and they will run down in 3 or 4 hours. there is enough power left to start the truck again. as soon as i start driving at highway speed the check gages light comes on and my voltage gage drops from 14v to 12v. the alternator does not put out anything. if i then shut the truck down and restart it drive slow (city ) or let it idol for 10 or 15 min. it will charge. is there a overload sensor that is tripping?

Answer
Hi Tyson,
Generally speaking, that not charging at RPM's sounds like a belt tensioner worn out, and the belt slipping.
I'd sure check that, but a worn tensioner would not explain the current draw when it is shut off.
A bad alternator could explain both problems, so I would be checking in that direction also.

Have you tried unhooking a battery cable, connecting a test light between the cable end and the battery post, and removing fuses, or disconnecting wires to eliminate, thus isolate the draw?

Van