12-3 tinned stranded wire will be fine. Anchor seems to dominate the market but any Marine UL listed will be fine. If it is only 50 feet look on Ebay - there are a few sellers of this type of wire.

Crimp ring terminals on the ends. And throw away all those metal boxes. Replace them with plastic ones available at any home improvement store.

The AC ground should never be tied to the DC ground - the AC ground should only be tied to the ground through the shore power cord.

Other than that it's all standard wiring practices.