Another no start post

I went to start the truck yesterday afternoon after work and went through the normal sequence of waiting for the start light to turn off and when I went to crank it, it tried turning over for a couple seconds then quit. A buddy tried to help me jump it, but no luck there. When it was hooked up to his truck, the battery meter would jump up to normal range then immediately drop back down to about 6V. If I put the key in and turn it to on when it is not hooked up to another vehicle, I can barely see the dash lights flickering but nothing else will happen. The dome lights are flashing on and off and I can hear the turn signal relay clicking on and off, but that's the only thing that it will do right now.

This morning I put it on a battery tender and it said the batteries were full. I also disconnected both batteries and put a voltmeter on them and it was reading 12V. Any thoughts on where to look next?