Originally Posted by J Birder
As a retired electronics engineer, I am telling you that you are giving out incorrect advice. Your formula is correct, but the wrong formula for the application. What you say is true for a motor driven device that wants to draw constant power. The lights and fridge are resistive loads. The formula that applies is Ohm's law I=V/R. When the voltage drops, the current drops also.
Additionally, since the OP is asking about problems in his 12V system, neither the size nor length of the any extension cord used to plug him in is likely to have any relationship to his problem.
Thank you for putting me straight. I honestly thought there was a motor in these things. Cheers.