Full sized trucks (as well as most other vehicles) will have 12V batteries that they use however it is simple to figure out what size battery you have by simply counting the number of cells or reading the label on the battery itself.
Each battery cell will be for approximately 2 volts which means if you have 6 cells then you have a 12V battery in that vehicle. If the battery that you have only has three cells then you currently have a 6V battery and not a 12V in your truck.
Batteries that have become damaged or worn out will typically not hold as many volts which is why as your truck battery ages your truck will become slower and slower to start before eventually not starting at all.