That diagram showing above does not appear to be balanced with regards to the (+) and (-) outputs to the RV from the two "12 volt batteries" essentially made up of two sets of 6 volt batteries:
The (-) output to the RV on the lower right should not have a short cable run to the right "12 volt battery" (made of two 6 volt batteries) because the (+) output to the RV on the upper right also has a short cable run to the right "12 volt battery". The (-) output to the RV should have a short cable run to the left "12 volt battery" (again, made of two 6 volt batteries) and a long cable run to the right "12 volt battery" - the reverse situation of the cable length runs to the two "12 volt batteries" from the (+) output to the RV.
The description above may sound complicated, but it isn't. Just duplicate the scenario of hooking up two 12 volt battteries in a balanced fashion. "Balanced" means that all currents into/out of the batteries must flow the same total cable length to each of the 12 volt batteries, so that neither 12 volt battery is used more than the other, whether supplying current to loads or getting current when being charged.
