I recently finished a big rebuild of my auxiliary electrical system.
I have been running all electrical accessories through a Bussmann RTMR for a couple of years, but my decade-old ARB single compressor bit the dust last summer and I took it as a great excuse to redo some things.
For starters, I had another mounting plate cut and set it up with two 40 amp MIDI fuses (the RTMR is limited to 30 amps per circuit) to power the ARB twin compressor I purchased to replace the single compressor. I also simplified the connections I was using and went from like six 2-pin connectors to only three, none of which can accidentally be connected to the wrong mate.
I moved the main breaker to the top of my battery. I countersunk some screws on the back side of my Slee battery hold down and used nuts to secure it in place, then built a set of new positive cables.
I then mounted the new panel and routed the wires through a hole I had drilled in the firewall, sealed with a grommet.
Everything comes through the firewall and is connected with a factory-style connector under the dash.
And of course I had to build the harness for the compressor, which is mounted on the other side of the engine bay. To simplify this, I had to take apart the ARB wiring harness and add a ground wire. The ARB harness comes out of the compressor as a five-pin plug, but two 10 AWG ground wires are combined into one 8 AWG wire, and an 18 AWG ground wire is expected to be grounded to whatever you're grounding the bigger one to.
I took that apart and went to two dedicated 10 AWG grounds, which should provide less resistance and works much better with the 280 Series Metri-Pack connectors I am using.
All of this goes to a panel on my drawer system at the back, which has the switch for the compressor, a switched USB port, a quick disconnect for the onboard air, and a battery monitor panel.
The next iteration will probably be a little different, I'm not totally happy with the wire routing on the panel I had made. It's not super space efficient and a little messy, so I'll revisit sometime in the future. But it works very well and is a better design than my previous system.