Thanks for your post.
All vehicles with a D4D will require a BCDC charger for the auxiliary battery charging, this is due to the temperature compensating system that reduces the alternators voltage as the engine reaches operating temperature.
These chargers are designed to be as close to the battery they are charging as possible, so with this in mind you will be best served by a BCDC in the vehicle, and a second one in the trailer.
You can use one BCDC if all of the auxiliary batteries are of the same chemistry and the cable between the batteries and the charger are extremely heavy.
You will need to use the BCDC1240 to ensure you have enough current to go around, and check that the auxiliary in the vehicle is capable of being charged with 40A, for when the camper is not connected.
Ideally, the BCDC's I recommend are:
BCDC1225 - 25Amp 3 stage charger & Isolator with MPPT solar regulator for the vehicle
BCDC1225 - 25Amp 3 stage charger & Isolator with MPPT solar regulator for the camper.
We have a couple of brake controllers to choose from called the EB (standard brake controller) and the EBRH (remote head brake controller), below is a link for the controllers.
If you require any further information please don't hesitate to contact me further.