If you park your vehicle for long periods of time, you'll find that your batteries will start to discharge, and eventually, dry out to the point that they need to be charged before the vehicle can be started again. The same process is true of diesel trucks, but they have multiple batteries, which can be a bit more tricky. There are a few ways to charge a multiple-battery system, but if you want the most complete and best charge possible, the following procedure a good option.
Pop the bonnet and locate the batteries. Disconnect the negative terminal on the first battery using an open-end wrench, then disconnect the positive terminal. Repeat this process for the other battery in the truck.
Connect the red (positive) clamp from one battery charger onto the positive terminal of the first battery. Clamp the black (negative) clamp from that battery charger onto the negative terminal of the first battery. Repeat this process for the other battery with the second battery charger.
Set the chargers for a low-amperage charge. The lower the amperage, the more complete the charge, but the longer it will take to charge. Turn on the chargers and allow them to sit charging the batteries for at least two hours.
Start the truck. If it doesn't start up right away or it hesitates more than usual, turn the chargers back on and allow them to sit for another hour.
Reinstall the battery terminals onto their respective batteries, negative terminal first, then positive terminal, using an open-end wrench.
There are companies that make automatic battery chargers that will turn off when the battery is full. Those types of chargers make the process a bit easier.