Our 21 has the dual 6V under passenger side dinette bench(as all do), however are next to kitchen wall if that makes sense vs. the rear corner as they have been doing for several years. The thinking is you get more storage immediately below the dinette bench that way, however it is a PITA to service batteries buried in the corner. It is easy to check water levels and even swap out as I did when trading the single 12V for dual 6V.
Pretty sure one could request that location vs. rear corner and for a 21 especially the loss of convenient storage given all it has anyway is minimal.
"We gotta get as far away as we can!"
- Russell Casse, Independence Day