All personal-use trucks made in the US are 12v. The dual batteries are needed because the starter requires much more amperage than one battery can safely delivery.
Most all diesels made these days have dual batteries, Ford, Chevy and Dodge.
Wired in parallel batteries keep the same voltage but double the amount of current and/or reserve capacity they have.
Wired in series, they double the voltage but do not increase the reserve capacity.
And if you really want to cook your noodle, I have six 6volt golf cart batteries in my camper, wired in both series and parallel.