NO 12 volt flooded battery is actually of deep cycle construction, except the Trojan t-1275/j150 or J185, and a few offerings from Rolls Surrette. They are all hybrid closer to a starting battery in construction than they are to a golf cart battery.
At best a 12v group31 battery is a marine battery with plate thicknesses half that of a T-105.
If you can fit three T-1275's in the space, they will likely be the most AH you can get of true deep cycle battery.
http://www.trojanbattery.com/pdf/datasheets/T1275_Trojan_Data_Sheets.pdfTrojan's T-1275 is rated for twice the cycle life compared to their own group31 12v battery. 1200 'Lab cycles" to 50% as opposed to 600 lab cycles.
http://www.pbase.com/mainecruising/deep_cycle_battery
