Battery Question

Our 2720SL (2006) came with a Kirkland 12V Deep Cycle Battery (Grp 24). Can someone recommend a good Grp 27 replacement that will fit into the same battery box?

Also, rather than try to modify the compartment for TWO 6V golf cart or two 12v batteries wired together, does it make sense to just keep the old Kirkland battery charged and in the back of the truck for backup? We generally boondock but no more than 2 or 3 nights out at a time. Our battery does recharge when we drive, too. (As long as the fridge is off)