http://www.ccis.com/home/mnemeth/12volt/12volt.htm
Most rigs that have 2 or more 12 volt batteries have them wired in parallel. when going to the 6 volters, you must wire pairs of them in series to produce the needed 12 volts. This is actually simpler than it sounds.... see the diagrams below.
When installing new batteries, first mark the cables so you do not forget which one is which when you reconnect. If you are changing over from a pair or set of 12 volt batteries to a pair or set of 6 volt batteries, some changes in cabling will be required. See the wiring drawing above for an example.. If you don't fully understand what the difference is between parallel and series wiring, I strongly suggest that you do not attempt to do the hookup yourself... get a competent RV mechanic to show you how. If you are building a bigger battery bank, see below for wiring info...
When replacing your batteries, remove the negative cable first because this will minimize the possibility of shorting the battery when you remove the other cable. Next remove the positive cable and then the hold-down bracket or clamp. If the hold down bracket is severely corroded, replace it. Dispose the old battery by exchanging it when you buy your new one or by taking it to a recycling center. Please remember that batteries contain large amounts of harmful lead and acid.
After removing the old battery(s), be sure that the battery tray and cable terminals or connectors are clean. Auto parts stores sell a cheap wire brush that will allow you to clean the inside of a terminal clamps and the terminals. If the terminals, cables or hold down brackets are severely corroded, replace them. Keep track of the markings you made on the cables before replacing them!
Thinly coat the terminals and terminal clamps with a high temperature grease or petroleum jelly (Vaseline) to prevent corrosion. Place the replacement battery(s) so that the cables will connect to the correct terminals. Be extra careful here, as reversing the polarity of the battery when connecting it may severely damage or destroy some parts of your RV electrical system. Replace the hold-down brackets or straps to secure the batteries in place, then reconnect the cables in reverse order, i.e., attach the positive cable first and then the negative cable last.
Before using the battery(s), check the electrolyte levels and state-of-charge. Refill or recharge as required.