You totally misunderstand.
The reduced wire size has nothing to do with AC vs DC, but rather is a function of the voltage produced. 120 VDC uses the same size wires as 120VAC.
A 12V (nominal) Alternator
prduces 12V AC before rectification. Transmitting the AC power will save nothing.
If your alternator is not charging you batteries
, are the wires properly sized for the amperage produced and distance traveled?? If they are, the problem is elsewhere.