A smaller size pop-up, without electric brakes, may only have relatively small wires running from the battery box. They may be as small as 12 or 14 gauge (more likely 10 or 12), and if they were never previously used, I'd look to see if they are taped-up somewhere in the frame rail near the box. As long as the positive wire end was taped (insulated) the inside lights, etc. would have worked with the 120v plugged in.
2014 RAM 2500 Laramie 4x4 Quad Cab w/Cummins Turbo Diesel, 2009 Rockwood 2607, and 3 'yaks!
300W of Suntech Solar, a Rogue MPT-3024 Controller, and a Xantrex PW2000 Inverter