24" x 7.5" x 9.5" tall
Looks like you are not going to be able to stuff much AH capacity there as it is not wide enough for 2 27's side by side, not tall enough for GC-2(t-105), or even a single t-1275.
You can't get a flooded/wet true deep cycle battery in the group 27 or 31 size format. Trojan makes both these sizes and they are just marine batteries and they are rated for only half the total cycles of a T-105. That said they are likely the best flooded group27 or 31 'deep cycle' battery available. With Crown, EastpennDeka and USbattery also making these sizes and being similar in weight and deep cycle durability.
But any other wet/flooded group27 or 31 like a wally world marine battery will not last as long as the above brands, but since they are significantly cheaper cycles per $ might be the same, especially if you can't really treat them well with prompt and full recharges.
IF you can handle the price tags of AGM, and understand they require higher amp recharge rates every so often and are more demanding of full recharges more often, then you can get a good deep cycle battery in there, such as a Lifeline.
You could get 2 group 24s Lifelines in there side by side for 160Ah total
http://www.lifelinebatteries.com/marineflyer.php?id=1YOu might be able to fit 2 Lifeline GPL-6ct on their sides for 220 Ah total.
http://www.lifelinebatteries.com/marineflyer.php?id=15Keep in mind Lifeline suggest a minimum 20% recharge rate when these batteries are cycled to or below 50%, and they want you to achieve and then hold 14.2 to 14.4 volts until amps taper to 0.5% of total capacity(20hr rate) at 77F.
Achieve this and these batteries will serve you very well. Do not achieve this recharge regimen and you might as well just get flooded group 24's in parallel and beat them up and just replace them more often. AGMs do not make for good wet replacements unless their charging parameters can be met by the existing charging sources, or the charging sources can be tailored to the AGMs needs
Always a good read:
http://www.lifelinebatteries.com/manual.pdfScroll down to page 20 for charging instructions.
I am getting good service from a group 27 Northstar AGM, but like Lifeline AGMs, it requires high amp recharge rates and being fully charged often. Slower recharge rates to full from a deep cycle, and the capacity walks down. Get to only 95% daily, and the capacity walks down quickly too. Luckily one or two high amp recharges from the most depleted state to bursting full restores performance, so far anyway.
NorthStar AGM's are sold as X2power at Batteries+. They add another year to Northstar's 4 year full replacement warranty.
Odyssey is another high quality AGM but they recommend 40% recharge rates when deeply cycled. 40 amps per 100AH of capacity. Kind of unrealistic to plan for this much initial charging amperage in an RV with a lot of capacity. The high amp rate need not be met every deep cycle, but it is a requirement every so many cycles.
There are AGM's that cost less and have a 30% maximum recharge rate, such as the Vmaxtanks and Deka intimidator and Universal battery, but these do not have the super low self discharge and the super high CCA ratings of the NS/Odyssey AGM's which might be beneficial under high inverter loads, if those are factors in your intended usage
So find your compromise since t-105s wont fit.
