I work on motorcoaches and this a common system. If you tap 12 volts for very long, like you are doing now, it will kill that battery even with the truck running. You would need a battery equalizer to do the job ful time. We use Vanner brand equalizers that are rated by amp draw. 24 volt fuel pumps are not hard to find nor are light bulbs. Make a friend in the local bus garage, they aren't cheap but certainly less than converting to 12 volts. If you need help with 24 volt bulb numbers e-mail me