It should work fine as you describe. Obviously you do need to have appropriately sized cabling with proper overcurrent protection for the connection between the camper and the truck.
It might help to think of it as two separate installations. You're hooking up the solar maintainer to the truck battery, and you're hooking up the isolator relay gizmo between the truck and the camper. If the camper is not hooked up, the maintainer is just maintaining the truck batteries, and possibly (but hopefully not if the control logic is well designed) powering the relay coil to combine the truck battery with an open circuit. The precise connection topology of the charge controller wires and the isolator wires is largely irrelevant to understanding what is going on.
I would suggest not hooking up the start signal to the isolator.
