Dang truck won't start.
I don't have a battery monitor, Oreillys hasn't had one in stock for some time (or the guy before me keeps on buying the last one). I have a new (reman) alternator which, when the truck is running, keeps the dash gauge right up where it should be. New Motorcraft GPR a month or so ago. New OEM glow plugs 18 months ago. New batteries (750 cca) 18 months ago as well.
Truck has been starting up just fine for the last two months, then last Friday I had to jump it to get going, after she sat for about 12 hours. Didn't drive it Saturday or Sunday, yesterday morning had to jump again. In the past, when I have needed to jump it starts right up, but yesterday it took quite a while. Even hooked up to the van, after getting a few minutes' charge, it still didn't start until after the glow plugs cycled off, then took off like a rocket. During lunch, after driving 20 minutes at freeway speed, the truck sat for about an hour. Engine was still warm, but she cranked really slow before finally catching. Charged the batteries last night, then this morning she wouldn't start - dash battery charge indicator dropped around 9-ish while GPs were running, then right up to about 12-13 when they cycled off. Still not enough juice to start the motor. Took the batteries in to Oreilly this evening, both passed their load test at 12.35 and 12.42 volts, both read low on charge, 70 and 74% if I recall correctly (doesn't surprise me with cranking on it this morning and cycling the glow plugs).
I slid under the truck and checked the nuts on the starter cables, everything's tight there. I'm charging the second battery right now, and will clean the battery cables. Again. No visible corrosion. We'll see what happens after dinner, but where should I be looking next? I have a cheap multimeter on hand, if that could be useful.

 

might want to get the starter checked

 

Maybe have the alt. checked?

 

I second having the starter checked. Starters can fail and give the symptoms of failing batteries. It happened twice to me.

 

One other thing to check would be the battery cables. Good and tight? Not just the positive but the negative also and not just where the cables connect to the batteries. Do you have a good connection where the negative connects to the block? Take all your connections loose and give them a little clean up with a wire brush or some sandpaper.

 

I've cleaned the battery connections (bristle brush, water, baking soda) twice within the last 18 months. I'll be doing it again tomorrow. I also picked up another tube of anti-corrosion "paste", I used that the last time I cleaned the connectors. Haven't checked the block connection, I'll hit as well. Thanks for the responses, all.