That DOES seem low to be a charging voltage, but it looks like the batteries are fully charged (you said 13.1V resting) and in that case, the truck may not be applying ""absorption" or "bulk" charging voltage, but instead is applying "float" voltage, which is usually just a bit above 13.0V.
Were the batteries being charged on shore power before you connected your truck? A battery in the "resting" state after the surface charge has worn off usually reads 12.7V if it's fully charged.
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH, Progressive Dynamics Converter
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew