I too am using the REDARC DC-DC charger to charge the secondary battery. I also have the Off-Grid Engineering dual battery setup. I have 2 Odyssey Group 35 batteries running my whole setup and have not had a single problem with it. I have the ML-ACR in the system to separate the two batteries so when I am camping, I can isolate the batteries and run all of my accessories off of the secondary battery and not have to worry about killing my starter battery, but if for some weird reason my starter battery is dead, I can connect the two so I can still start my rig. I have had no issues with the stock alternator keeping the starter battery topped off, even though everything I have read says that it isnt enough. When I installed everything, I used circuit breakers instead of fuses due to the ability to just reset them if they pop. I have them everywhere so in case there is a short in a wire, it wont effect either one of the batteries. I have ALOT of stuff running off of my secondary battery for camping besides all of the auxiliary lighting like 2 'power stations' in the bed of the truck that have USB outlets for charging iPhone and bluetooth speaker, a couple of 12v outlets because my RTT has USB ports in it with a LED light strip, and I have a Dometic fridge.
If you have any questions, dont hesitate to ask. I had to figure all of this stuff out for a dual battery system myself because there just wasnt enough information when I did it and everyone had their own theories and reasons they did what they did. I bought alot of stuff and ended up replacing it with something else because it didnt do what I thought or was told. It was a process, but I will try to help you out if you have any questions.