Sorry you're having troubles. When not hooked up to shore power, the 12v power is provided by the battery(s). What is the voltage output reading of yours? Is anything preventing flow from the battery to the inside fuse panel distribution center like the salesman cut-off switch or are any of the large in-line circuit breakers or fuses tripped?
If battery installed backwards or wire grounded even momentarily, likely breakers near battery tripped, or larger main fuses in POWER CENTER "REV POLARITY" are blown?