Maintaining a 12v system from your 24v vehicle
I am using my deuce as a camper, and need 12v in the back for lights, radio, etc. What I ended up doing was to run 24v to a solar charge controller, which does a full 3-stage charging to my 12 battery bank. The charge controller must be a pulse width modulated style, and NOT a shunt. I used a Xantrex (Trace) C-60 model. It is rated at 60 amps continuous. Depending on your needs, they also make a C35 rated at 35 amps continuous. There are other manufacturers also.
The system is a little more complicated than that, in that I ran the truck's alternator feed to a battery isolater to separate the two systems, which prevents draining the truck's batteries, and prevents heavy surge currents from the truck's batteries. I also put a 100 amp relay in the circuit that goes to the 12v charge controller, so that I can switch it off if needed.