I also have the D-180 panel. I simply moved the wire feeding the 12V power point from the hot side of the master relay to the switched side, and replaced the 12V power point with a dual USB power plug. Now it's switched on by the master.
I'm sure there are other suitable ways to do it, but that's what I did.
RV-12 # 222 N980KM "Screamin' Canary" (bought flying)
Fisher Celebrity (under construction)
Previous RV-7 project (sold)