Battery cable size

Bigger is better, go with the Welding cable...

You can get away with 1/2 the size of the trailer feed, assuming we're talking about 12v batteries.

I agree with Time2roll.

Same as the main RV cable connection. Probably #6.
I recommend the main ground to one battery and the main positive to the other.

Pro looking custom cables here:
http://www.genuinedealz.com/custom-cables?size=56

Most battery studs are 5/16" to get the right lug.

I made up my own with welding cable (available by the foot at any welding supply place and crimp on solid copper lugs, also available at the welding store.