charging of 2 camper batteries with truck alternator?

Good luck getting camper-specific answers from Ford.
What I think about heavy cables connected by straight relay to truck system is that when you have dual batteries in camper depleted to 10V and the relay kicks in - the camper batteries will take truck system down to 10V and kill the computers.
For this you sure want battery combiner that will protect the truck.
The small factory wire protects the system by its own resistance.