Heater Ignitor Works On 110V But Not Battery

I'm out at the camper right now working on this problem. The fan is noticeably faster running on 120 volt shore power. The battery is fully charged and the voltage hardly drops when the fan kicks on. I ran an external ground from the battery to a jack with jumper cables but that didn't help. I also ran it directly to the heat frame and it started to smoke. Next question, and excuse me if this is dumb. Could the battery be hooked up backwards? I bought the unit with this problem and here is what I'm finding. There is a black cable going to the (-) on the battery and a white cable going to (+). Tracing the cables as best I can (they disappear for a moment behind a cabinet and I can't see them) the black cable from the battery runs to the inverter. The white cable disappears and I can't trace the entire thing. All I know is at the jack mounting bracket on the outside of the camper I have a white cable (very similar to the one hooked to positive on battery) and a green cable that comes off the inverter. Could this white cable be the one on the (+) of the battery? If it was hooked up backwards would anything actually work?