There is no easy way to do this while maintaining the solar charge to the truck. The most reliable way would be to run another wire for the solar charger, and have the main wire for the DC-DC charger be switched.
A '14V relay' is not going to work. First, you would want a Voltage Sensing Relay (VSR) not a '14v relay'. Second, when your solar charger is charging your truck battery is will trigger the VSR, which will start your DC-DC charger, which will pull your truck battery down and it will likely end up in a on/off cycle.
One other question - do you really need the DC-DC charger? What sort of charge current do you get with out it?