It is not a good idea to wire two batteries which have different voltage ratings in parallel because the one with the higher voltage will immediately send current to the other one with the lower voltage to try to make their voltages both the same. The very high current which will flow between the two batteries is likely to make the batteries get very hot. IF THEY ARE CAR BATTERIES THEY COULD EXPLODE!
When connecting two batteries together on an RV trailer you usually connect the batteries in parallel to draw 12 volts from both of the batteries. We have a photo, see photo at right, that shows how to wire the batteries in parallel.
Once you have the batteries wired properly in parallel you will need to connect the 12 volt power wire from the trailer 7-Way connector to the positive wire on the first battery. This connection will help maintain the batteries when the vehicle is running, but will not charge them sufficiently from a discharged state. You should be sure your batteries are fully charged before leaving for a trip.
You will need to disconnect the trailer from the vehicle when parked for extended periods, or use a battery isolator, item to keep your vehicle battery from being discharged if the trailer batteries are being used.
To wire the Battery Isolation Solenoid you will need to splice the side terminals inline with the 7-Way Power Wire, attach a wire that is powered when the ignition is turned on to the small post on the front of the Isolator, and then mount the isolator to a portion of the vehicle that is grounded to the vehicle frame. I have posted a photo of a wiring diagram for the Battery Isolation Solenoid for you to review.
Since the Outback FX2012MT inverter is a 12V inverter, I had to wirethe batteries together to make up a 12V battery bank. To start with,each battery is 6V. Just like with solar panels, connecting batteriesin series (positive to negative) causes the voltage to add while thecurrent (AHr) stays the same. So I connected the positive of one 6V, 225AHr battery to the negative of another 6V, 225AHr battery toend up with a 12V, 225AHr series string of batteries. That gave memy 12V. So I repeated this with the remaining 6 batteries giving me4 separate 12V, 225AHr series strings. Since connecting them furtherin series would give me too high a voltage, all that was left was to connect these stringsin parallel. Connecting in parallel leaves the voltage the same butthe current (AHr) adds. So now I had 4 parallel connected stringsfor a total battery bank size of 12V, 900AHr (4 x 225). UnfortunatelyI was too busy installing the system to remember to take a photo butthe following diagram illustrates what I did.