I've seen a couple of different versions of the aft cabin setup, everything from bunk style beds to an island bed on the *centerline." The most common is the twin full size beds on opposite sides with two 125-ish gal tanks under each. Some boats only have one 125g under each bunk. But if you're not a long haul boater, then 250 gallons will take you a long way.
The fuel tanks are above the aft cabin floor, so if you modify the beds, you have to modify the fuel tanks too. The island bed arrangement, we viewed had twin tanks, still under the bed. We didn't like the setup, so we didn't actually inspect the tank(s), but would have no doubt, resulted in less gallonage of fuel.
The island bed also limited the access through the aft cabin hatch. You're standing on the bed to enter/exit the hatch.