I can only assume that you want to wire an INVERTER and not a CONVERTER. A converter changes AC to DC and USUALLY it would also step voltage down to a lower rate; e.g. 110 AC to 12 volt DC. These type of units usually come with the 110 AC plug installed. For an INVERTER, you would usually be making 110AC from 12 volt DC. That requires some very heavy wiring directly from the battery box. You will want a fuse inline, but no other breaks in the main power supply wire or the grounding cable. For a distance of 12 ft I used a 1/0 or a "One OUGHT" cable when I put one in my Semi truck. For safety and longevity of the inverter, use the biggest wire you can find. The same size that is used on your home by the power company should be sufficient to go from the towing unit to the back of ANY 5th wheel trailer. Be sure to tighten the connections again after the first few uses, as these stiff wires will soften under the heat of a load.