Truck started at 13.4 volt. Got going down the road & is now 11.7-12.5. Pulled over check belt & connections seem fine. Multmeter reading 13.7 at alternator. Dash start showing 13.4. Get going again. Start reading 11.8. Pull over & check alternator is 13.7. Batteries all tight. Am I missing something? I'm 300 miles in my trip with 300 more to go. 07 freightliner columbia with 60 series 14l.