Nothing wrong with adding a high powered DC-DC converter like OP is doing. I like the contactor for auto power off, just make sure it is continuous duty rated (like the one in our PCB for dash power).
On the same token, and a lot of people have a hard time with this one, is that the stock military M939xx wiring configuration is to draw 12v from one battery (pair) only for the heater blower fan low speed. It is even separately switched as a dedicated 12v circuit through the dash switch. I know a lot of people will make several excuse-guesses for why they did it, but there it is, factory designed, approved, and released build configuration, in the wiring diagram and every 939 made. The argument becomes more difficult once done for production.
Personally, I treat batteries as consumables, as I run 2 x new $100 group 31's and would draw as much 12v off one battery as needed. I toss them at the 5 year mark and replace them regardless of how they test or crank the truck anyway, judged by the calendar only. I think it is cheaper to do it this way rather than spend extra $ on a 12v system. To me, it doesn't matter if the 12v from one battery is harming it or not, as it is getting tossed out sooner rather than later anyway. I have done it, and have not noticed any adverse effects.
