I have an AGM battery in the tub, it was being charged by an isolator, but that was not ideal. My sparky had a secondhand Redarc DC-DC at a price I could not pass up; so I installed that and disconnected the isolator.
Doing this myself, I found that DC - DC is the best setup (no voltage drop issue, cable choice is cheaper), the only concern is the right cable to the battery charging (can only be 600mm - 1 metre) from Redarc DC - DC charger.
If I was to run a battery in my camper trailer (which I am), I would use the anderson plug setup from second battery, to a DC - DC charger in the trailer, set up the same way as the tub in the MN. Once at camp, I would disconnect to save second battery discharge.
Re: anderson power...I have run from both (starter battery under the car, through chassis/brake lines, as I run an air compressor and dislike popping the bonnet to power. It seems like overkill now, but I can connect solar panels to either and charge, so will leave as it is. If I was in your position, I would just run the anderson plug from your second battery - as the CTEK (if the same as Redarc) will take a lower voltage and charge at appropriate charge. I have tried this to test the second hand Redarc, and it worked fine. All cables are fused with 40 amp midi fuses - at both batteries. Even anderson plugs installed in my setup are fused, as I do not want my dog to die from fire.
I was lucky getting a Redarc charger for a third of the price, and setting it up was easy. I will hunt for another one for the trailer; but am not in a hurry - as I am super busy with work for the first part of the year.
Hope this helps