You have several options to obtain 12VDC power in your 24VDC truck. Here are some:
> Replace your existing generator/alternator with a duel output (12 & 24) HMMWV alternator (This may be the most expensive option.)
> Add a second 12 volt alternator as mentioned above
> Add a battery equalizer and a third, dedicated 12 volt battery
> Use a power converter
> Center tap one of your existing 12 volt batteries (not recommended)
I only wanted to run a few accessories (e.g., cell phone chargers, GPS, etc.), so I went with a Pyle converter. There are details of my installation here:
http://www.steelsoldiers.com/showthread.php?120966-My-12-Volt-DC-Accessory-Kit-Installation
There are a bunch of threads on this site about 12 volt power. With a little research, you will be able to find the best solution for your specific needs.