I think the run from the C-Tek to the caravan battery would be a little long. It's still doable like that, heavy cable would minimise the problems.
When I first started putting batteries into my caravan I used mixed sizes and found that troublesome because the smaller capacity battery would charge first, leaving the larger capacity batteries wanting more. When the charger came off, the smaller capacity battery would feed the bigger one so you'd end up with an undercharged pack.
If your aux and your van batteries are the same capacity that too reduces problems.
When (if) we get a new caravan it'll have a big 3-way fridge in it (our current van has a 12/240V fridge with a compressor). To run the 3-way fridge while driving, the accepted practice has been to use 12V fed from the tug through an Anderson plug to provide 12V power to the fridge.
Unfortunately I've learned a little about how those fridges work. On 12V, they only just barely operate the heater, they don't regulate the power (there's no thermostat being used here) so it will only just maintain the cold on a reasonable day. On a hot day you're losing. The thermostat works on gas or 240V.
Since you can't use gas while the vehicle is moving, the logical choice is to invert power (you need an inverter capable of supplying a constant 400W) and then you have a fridge that will be able to cool things back down after you've stopped for lunch here:
Okay, so if you're already inverting power for the fridge, maybe it's time to go whole hog and invert power for the battery charger as well? A 15A charger will consume around 20A of input power, plus 400W = about 33A so 50A of input required. Maybe some bigger Anderson plugs are needed?
My van above has a 7A C-Tek, 1.6A Projecta and a Waeco fridge that draws about 78W while running (6.5A @ 12V). Total draw on mine is less than 20A - but I'm getting my car ready to deliver 60A to the tail if needs be.
Only problem is getting the alternator to deliver that constantly!
