For your further education...
There is one item that runs completely off the trailer's battery- the breakaway switch. Should your trailer ever completely disconnect from the tow vehicle and become a runaway, the plug will be pulled out from the switch and the trailer brakes will be applied using the trailer's battery for power.
I periodically pull the plug (when the trailer is parked and wheels chocked) on the breakaway switch to ensure this safety device is working properly. If it is, you can usually hear the brakes click, hum or make some kind of noise to show they're engaging. If you don't hear anything, have someone put their ear close to the wheel(s) and listen while you pull the plug. When the test is completed, re-insert the plug back into the switch.
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TV: 2009 GMC Sierra 2500HD | Crew Cab | Std. Box | 4WD | Duramax/Allison
RV: 2000 Jayco Eagle 266 | FBS | TT -- 1986 Coleman Laramie pop-up -- Still in the family!!!