You are correct 12 Ga is way too small abandon it. I think this is the most important obstacle that you will have to overcome with RV Solar. Just in case you haven't seen these sites before I have included some links:
http://www.amsolar.com/
http://nooutage.com/vdrop.htm
http://www.where-rv-now.com/Notes/Solar/index.php#Calculator
http://www.rvsolarelectric.com/index.php?route=directory/directory&directory_id=3
http://www.jackdanmayer.com/rv_electrical_and_solar.htm