Under 1/4 berth
I have group 24's under the forward end of teh 1/4 berth. There's lots of room there, and while it doesn't "vent" in the traditional sense, in an outboard powered 27 (which it sounds like you have), any fumes have lots of room to move back under teh cockpit, since the underside of the 1/4 berth is open to the "garage" under the cockpit (where an IB would normally be - it's where I keep the inflatable and extra sails) the 1/4 berth has lots of room for any fumes to dissipate. Only problem is weight distribution. With the 27's OB all the way aft (85lbs), two 8-gal fuel tanks for the main OB (180-200lbs when full), the radar and mast (70lbs), dink under the cockpit (75lbs), the dink's OB on the stern rail (30-60lbs depending on which OB I bring), anchors & rode in the cockpit locker (50lbs), and 3-4 200-pound crew in the cockpit, I'd rather the battery weight was further forward. Some have put them under the V-berth, but then they're harder to get to, and cable runs are longer.