Sounds 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.