Trailer wiring help

The problem, as I see it, with the OE tow vehicle charging wiring is that the wire is too small for the length of the run (have to include the return or ground length as well). Results in excessive voltage drop and the trailer battery never really gets fully charged. When that happens the trailer battery doesn't live very long and performs poorly during it's life.

The max wire size that any RV light plug that I've seen will accept is 10 ga., and that's usually for the electric brake circuit not the charging circuit.

When looking at this sort of thing it's really easy to find ampacity charts for AC, but DC is different, it's ampacities in a given gauge will be lower since AC has a cooling effect due to it's cyclic nature. Look at this chart:
http://www.polarpowerinc.com/info/appendix.htm#Table C-2
Note that it is for one-way so it's 30 foot length limit of 3.5A in a 10 ga. wire would be only a 15 foot run. Which is shorter than most trailer charging distances. That's for a 2% voltage drop, which isn't a lot until you consider that a 2% VD drops the charging voltage to 13.33 volts if it was the usual 13.6VDC alternator output (some go to 14.4VDC). There needs to be a charging differential, the charging voltage has to exceed the battery's standing voltage by some amount in order to charge the battery. As soon as the charging voltage isn't high enough, charging basically stops. That seemingly tiny voltage drop in the too small of a wire just short changed the trailer battery from getting a full charge.

The reason to run a large gauge cable for trailer battery charging isn't because you're going to be pumping 100 amps thru it. The reason is to have the tiniest voltage drop reasonably possible.