That’s correct Robert, the DC-DC charger should be placed as close to the battery or batteries it’s charging as possible. After boosting the voltage to the optimum level for charging, you wouldn’t want to introduce voltage drop from too long a circuit.
There are at least three brands you can consider for a DC-DC charger: Redarc, CTEK, and Renogy. I’m only personally familiar with Redarc, and can recommend them. I think you’ll find the best price is at etrailer. Don’t undersize the wires between the truck batteries and the DC-DC charger or you’ll be disappointed with the performance.
:):)