Hello Bl_smith25, we have a 2006 Sprinter / 2007 Pleasureway conversion with the house battery (Grp 24) under the floor at the back. I need to increase the house battery capacity from the Grp 24 ~75 amp hr to something over 100 amp hr to be able to run a cpap overnight (7-8 hrs) without drawing down the battery too far. I've been thinking about replacing the battery box in its current location with one that is large enough to hold a Grp 27 or Grp 31 battery. There appears to be enough room to fit a box for a Grp 27, maybe even a Grp 31. But this means removing the current battery box which is welded to the frame (a knarly task) and installing a new one for the new battery. I like your idea of installing a second battery on the floor above the current one. You give up some under the couch storage but keep the re-cabling simple. Would you post a couple of pictures of your installation? I'd like to see your cable runs and how you've secured the battery box. TIASounds like you have a 2006 with the battery under the floor at the back. It is not a good location to add another battery under the passenger seat with this setup since the two batteries are so far away. They'd need to be two different sizes and the proper cabling between two different batteries at that distance is too complicated to get it right for equal charging and discharging.
Consider adding a battery over top of the one under the floor. Set it on the floor under the couch as far forward as the wood cabinet will allow. From there, the wiring is easy and you can do either two 6V in series or two 12V in parallel and get the wiring right. You MUST use AGM batteries for both of course. The original group 24 battery under the floor was AGM as it's considered to be in the living quarters and if you add one on the floor under the couch, it certainly is.
I have a group 24 in the box under the floor and a group 31 on top of it wired in parallel. While not ideal due to the mismatched sizes, the cabling and connections are short and easy and correct. They've been working well for probably around 5 years now.