Hi,
Most alternator wires are a bit undersized, and I think one of the reasons is the auto manufacture really does not wany the alternator to put out a full 14.5 volts and 130 amps. So they use #8 wire from the alternator and run about 8 or 10 feet of wire before it gets to the batery. IT has the effect of limiting the battery charge rate, and that is probably a good thing. I found that when I connected some #4 wire to my alternator and a 125 amp solinoid that is connected to the coach battery, when I turn on that solinoid, then the alternator belt starts to screem "Help". So normally if my battery bank is a little low, I leave the switch off until I am at least out of the camping area, and at freeway speeds. At 800 RPM, the alternator belt is only going about 600 feet per minute, but at 2,000 RPM, it can be going 2,000 feet per minute, so there is less pull on the belt - because the torque is spread out over more feet of belt.
So #8 wire to the alternator will provide the best voltage to your battery bank, while not overloading the alternator to much. I used a forklift relay, rated at 100 amps with silver allow contacts, to isolate the battery from the engine battery when the engine is off. I control it with a switch, because most of the time I don't need to use the engine to recharge the 400 AH battery bank, my 400 Watt solar system does that.
You should run #8 wire both ways, it conducts power better than steel. What I did with my inverter, that sometimes sees over 150 amps into it, is run #000 wire to a two lug terminal that is bolted to the frame, then to the negative to each battery (I also installed a shunt to measure any amperage being used out of or recharged into the battery bank.) So power can take the lease resistant way to make it's way back to the alternator via the frame of the RV, or to the battery via the copper wire thick as my thumb.
You also need to consider fuses. I would recommend a 50 amp automatic reset fuse for #8 wire. This should also be located near the output of all your coach batteries, and near the other end of the wire, the alternator, or battery connection to the tow vehicle. If a short happens in between a pair of batteries, power can come from both directions, and start a fire, while the circuit breaker will overheat, and stop the power flow for a little while.
Yes you will be mixing a pair of 6 volt batteries, and some 12 volt batteries, this is what is in most motorhomes that use 6 volt batteries, and they get along fine.
Fred.
