Triple battery setup run down.

[Cuppa]

I've got mine much like you are describing only I have a crank and aux on the car and two aux on the camper, both on the trailer bum off the aux batt on the car via Anderson plug and I've never had an issue. I've also got a 120w panel for longer stays with plans to purchase a second 120w

Edit:I use a ctek 250 dual on the car

I assume you mean that the two camper batteries are connected to the aux under the bonnet via the Ctek?