Although people often claim differences between 6V and 12V batteries, they're usually comparing apples and oranges. You can buy batteries in 2V, 4V, 6V, 8V and 12V which are constructed exactly the same way, with the same performance. If you can find the right sizes, you can build up a 12V bank from three 4V in series, two 6V in series, or one 12V... and double the capacity of any of them by adding another set in parallel. The construction matters, but how many plastic boxes you split the lead plates up into doesn't. As Jon mentioned, desirable batteries in 12V are
available, just not as common as 6V and thus expensive.
There is a legitimate difference between series and parallel. If you resort to putting two 12V batteries in parallel because one is not big enough, the two batteries don't share the load perfectly, which doesn't help reliability.
Usually, people are comparing relatively inexpensive consumer-grade 12V deep cycle batteries (two of them in parallel) to commercial-grade 6V deep cycle batteries (two of them in series), to conclude that the 6V are more durable.
: for RVs, the popular setup is two 6V batteries in series, in a size referred to as the "golf cart" size. I looked for data about golf carts (which are actually called golf cars in the industry), and all three of the manufacturers I checked actually use six 8V batteries to form the 48 volt bank that they need, not eight 6V batteries... those golf carts don't use the "golf cart batteries"