2oldman wrote: that depends on the size of the cables feeding the RV.
No it doesn't. It's going 6 inches and voltage drop is practically nothing. 8 ga is plenty. I noticed my Progressive Dynamics 4600 series uses 8 ga to the fuse board but to the batteries is what matters. Bigger the better on a longer run.
The same size as the positive and negative cables going to the RV. All three will carry the same amperage.
When I switched from two 12 volt batteries in parallel to two 6 volt batteries in series, I used one of the two short cables that had previously connected the two 12 volt batteries together.