The voltage of an RV battery is not a matter of better or worse. It's all about the amp hours delivered and how long the power will last to run your appliances. However, it's very common for people to have a misconception that the higher voltage (usually 12 volts), would be more powerful and would give them a longer charge when in reality, both 6-volt and 12-volt batteries can give you essentially the same amount of time before needing another recharge. What makes one better than another is simply the size of this type given its corresponding values: as with cars, bigger means stronger (i.e., more miles=more space for energy). So its all up to what features work best for their situation rather than going by the voltage. You can also wire lower voltage batteries in a series to reach a higher voltage, like 12 volts. However, you can't go the other way and wire batteries with different voltages in series as it may damage each battery and destroy the whole set up. The only time you'd see an RV with two or more separate 6-volt batteries is when they're connected in parallel to create the 12 volts, which is better in terms of weight and cost.