I let my truck sit for a couple of days (4) to be exact over the long weekend we had in Oklahoma because of the snow. When i went out to fire it up this morning it barley turned over the 1st time. I cycled the key twice again and then tried again it turned faster but still not close to starting. I waited a couple of minutes and cycled and tried again it cranked slow but fired off. When the truck started the voltmeter was charging halfway between none and normal i figured it was the grid heaters cycling but i never heard them. I don't think that would have anything to do with the truck turning slow but what the heck might as well check it all while i'm there huh. The alternator was tested a few months ago and it was fine.The batteries are new one 1 yr and the other 1 month. 925 or 975 cca i'm not sure but more than the stock either way. if someone knows where the grid heaters and relays are and how to check if they are working. When you start the truck if you are holding a tester on the batteries do both of them pull down.

 

You might have a draw or bad battery. The way they are hooked up a bad one will draw down on the other one. The grid heaters draw abot 200 amps as i recall as they are cycled.

 

When the grid heater are on my charge meter is about the same as yours,
sounds like a battery drain or the cold weather killed one of your batteries.