Depends on how efficiently you want to charge. I have three deep cycles with a constant duty solenoid used to connect them to the alternator.
I have a heavy cable (0 AWG) going between the alternator and the solenoid and then a marine 100 amp breaker on the line going to the Deep cycles from the solenoid.
I used #4 wire for the charge circuit to the batteries so that they could recharge properly. With low voltage, you need heavier cabling to reduce the voltage loss caused by load.
In order to facilitate this heavy wire, I installed a separate Lift-Gate plug and cable that's rated for over a 100 amps, so it wasn't any harder to hook up than pluging a standard 7-pin.
Also, I put my camper plugs in the front of the bed on the driver's side to shorten the amount of wire needed for the circuit as much as possible.
