@Pucker5 , the Redarc doesn't discharge the battery at all, just senses aux battery voltage and decides what it needs--it has different charging profiles you can select based on your aux battery's peak charging voltage. I'm on profile A, which maxes out at 14.6V, IIRC, and delivers three different types of current to the aux battery--boost current which is pretty strong and continuous voltage initially, then another one I forget the name of that gets the aux battery up to 13+ volts, then a "float" stage that gives it periodic little sips off 13.3V to keep it completely topped off. Apparently more effective for getting a full charge than a relay that effectively just directs full alternator voltage to both batteries or isolates the second one--my mad scientist friend says that'll never get a deep cycle battery charged much above 80% capacity.
@Dparo, I couldn't find a ready-made Redarc mount. Longranger has one for the 100 prototyped but never got back to me on when they thought it'd hit market. So I bought Lowe's beefiest 90 degree shelf bracket and, after experimenting with a couple different approaches, realized that I could just cut one leg off, drill three holes in it (2 for the Redarc on each side, plus two mounting holes) and mount it to two existing, threaded M8x1.25 holes. One of those is where my horn was mounted, so I just discarded the plastic horn spacer and rotated the horn about 40 degrees to the left, as I think you can make out in the photo. The other one was just not used. Stacked some washers up to get the bracket away from the lip of the radiator support and it works pretty well. It's super solid, but not that elegant.
And by tray do you mean the Blue Sea fuse box tray? That's a late model Taco bracket, IIRC, from Overland Equipped that was super easy to adapt to the 100. If you search my posts you should find a short writeup--I think on this very monster thread.