Depends what you're isolating.
If adding a battery to the trucks single battery, you can but don't need to isolate.
If you're using the 2nd truck battery as a house battery with a dedicated charging circuit or link to the camper battery(s) then I'd definately want to isolate it from the start battery.
If you're talking about adding a 2nd house battery to the camper (similar to in the truck), no need to isolate them from each other. Dedicated chargin circuit from the truck I'd isolate.
Regardless, I've had good service out of the blue seas ACR I put in the boat.
Never touch it. It opens when it senses charging voltage and closes when it gets below 12 or something safe to unhook the other battery before it is discharged.
No fuss, don't need to do anything it latches and opens as needed.