I sent a email to ZVOX and got this quick reply, much appreciated for a Sunday:
"The Mini comes with a 120 VAC power adapter (in line type, cords at both ends) which provides 13.5 VDC/1.8A to the Mini. It's a iron-core transformer supply. I don't know the idle power draw, but my experience with similar supplies is that it is around 5-7 watts. A Mini playing at full volume is unlikely to exceed 15 watts draw over time spans exceeding a few seconds, simply because audio isn't constant. Peak draw could be much higher (40 watt as a guess), but this would be very brief."
Now I'm no electrical engineer, but with a 1.8 amp draw at 13.5 VDC, it looks like it might be able to be run directly (but fused) off the trailer's batter[y]ies without using the AC wall wart. A non-peak operating draw of 15 watts at 12 VDC should be only 1.25 amps (15W/12V), which sounds pretty efficient. Guessed peak draw of 3.33 amps (40W/12V) doesn't seem bad either. Looks promising; party on!