There are more things to look at, like charging series connected batteries vs parallel connected batteries... the extra battery lead (circuit resistance)
needed to complete the 12 volt parallel battery connection. These may seem trivial but over time they play a part in the life cycle of a battery system.
The easy answer is that in a flooded battery system, there are less caps to check on a 6 volt system (6 caps - 2 batteries) vs (12 caps) on (2) 12 volt batteries.
To a normal RVer, the above is way past the information that they want to know, so they just purchase what ever the dealer makes the most money on, as in most cases they have no clue either.
FYI.. Any battery with CCA, CA, MCA, or MC is a dead giveaway for an SLI battery (Starting Lighting & Ignition aka car and truck batteries) and is
NOT a True Deep Cycle Battery
If you are interested here is Trojan's product list:
https://www.trojanbattery.com/pdf/ProdSpecGuide.pdf