Very nicely done.
Dual batteries is coming up on my to do list. You've given me some more ideas.
How is your system different than the NL kit other than your "brain" can handle 300A where it looks like the NL can only take 85A. Correct me if I'm wrong there on current loads. Also did you hook up any in the cab controls for switching and/or the LED light described in the pdf for your relay?
I like the idea of building my own kit too as I'm heading in the direction of placing my 2nd battery under my truck using the Metal Tech battery tray that bolts up to my sliders. Keeping my center of gravity lower and less crap in the engine bay.