comments:
Move the ground wire for the charger to the last battery. It balances the load better across the bank. And make sure both wires from the charger are the same length. I had problems with keeping batteries balanced in the RV when I had it wired like you do. In theory since your going to run a 12 volt AC system, you should have wired them in series for 36 volts to get maximum battery life. High draw, long run time loads are always on high voltage setups. E-Bikes are a perfect example of this. Also, if you are going to tap the hotwire from the tow vehichle to charge while you drive you need a 12 volt to 12 volt charger. I wouldn't go higher than the 9 amp charger without making sure you sufficent wire size to carry the load on the factory wiring.