TL;DR - Even with relatively cool days and nights, something was wrong with my house battery. Need to investigate.
I have a confession to make - a few days before departure, I left the ignition on in the truck overnight, and that meant that the radio, clock, and maybe a few other little things were on for ~24-36 hours. My batteries didn't love it, and the draw was low enough that the Blue Sea ML-ACR (automatic charging relay/battery controller) did not isolate the batteries at any point. When I discovered my mistake, the voltage of both batteries was at something like 12.1 volts, and I let the truck idle for 30 minutes or so just to get things back to a happier place.
I figured that the 18 hour drive south would be plenty of time for the alternator - which hums away at 14.5-14.7V - to get both batteries back into primo condition for the remainder of the trip. But that was not the case. The starting battery seemed to recondition well, but the house battery didn't. I would get up to a full charge, but it also seemed to lose that charge more quickly than usual.
I plan to get a CTEK 56-959 battery conditioner/charger and see if I can recondition it properly. If not, it's still under warranty, so I'll need to look into that.
On the plus side, having two batteries meant that I could still isolate the starting battery in camp at night, and not have to worry that I wouldn't be able to start the truck in the morning, so that was a huge plus!
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