Auxiliary Battery Wiring for Portable Fridge

I have to use the factory plug or cobble up a new connection for the fridge. Could do but don't wanna. Factory wire will be shortened somewhat when adapted to battery screw posts, maybe 6'. Aux outlet to battery will be less than 3'. For these wire sizes: With my alternator cranking out 14.2v and charging the battery at 20 amps (more likely less than this) the voltage at the battery would be about 13.6v. If the fridge is running off the aux battery 12.6v, the voltage at the fridge would be 12.3v.