Just do as ProfDant said and then attach the batteries in series rather than parallel to get 12VDC. Use a heavy gauge conductor between the two batteries and put some tape on both ends that is neither Red nor Black (perhaps blue) just so you or others know the difference. I imagine you are using deep cycle batteries so you need to make sure your 12V charger is correct for deep cycle batteries as the charge voltage is higher than for normal no-maintenance batteries. Technically speaking, your tow vehicle will not properly charge deep cycle batteries and they will be undercharged after driving.
2003 F250 7.3L Powerstroke Crewcab longbed