4 6 volts _ I need 12 v output for auxillary device

I can answer your question, but I need to make a quick statement.....you did not give me the current rating of the batterys.
The circuit you described gives 12 volts @ a maxium current of X (unknown)
Just tap the two outputs (#1or #2) & (#3 or #4) you will get the 12 volts you need.

You appear to have a 12 VDC system for your RV already.  You have two sets of 6 volt batteries in series for 12 VDC in parallel with another pair to give the motor home more reserve electricity when you are not running the engine.  Simply make your connection to one of the two negative posts connected together (but NOT connected to a positive post) and your other wire to a positive post that is connected to another positive post (but NOT a negative post).  You should always have a fuse in series with your new device on the positive side.  It is common practice to have metal in most vehicles connected to the negative side of batteries and if your positive wire touches any metal, BAD things happen... If unsure of what you did, just connect a 12 VDC light to this connection instead of expensive electronics.  Normal brightness would mean you did just fine.