Might use a voltmeter, and check out. Connection is under USB port, normally just remove 2 screws. I’m adding a extra higher capacity usb double port, with digital 12 volt charge display, inside new North Trail. No outside USB port on mine, so will add a 2nd port, to lid of 2nd battery box I just added. Removed small Interstate battery that came on tongue, and put 2 large AGM batteries in, since fridge is all 12 vdc. Top of battery box, will be a simple install for double high cap USB port/ voltmeter. Probably put lit waterproof switch next to it. Can’t shut fridge off so a switch there too. On my 2nd week, fixing stuff overlooked, or installed poorly by factory. Since the tools are out, I’m putting some extras in, as time permits.