In a 2720SL, the battery is located in the rear compartment, accessible from outside. The battery box is recessed into the floor, and is big enough to accommodate either a Group 24 or a Group 27 battery. I believe that the battery is added by the dealer, not the factory, but I don't know who makes the cutout in the floor. It is worth noting that the weight of the battery is supported entirely by the thin aluminum outside skin of the floor, since the upper aluminum skin and foam core are cut away.
To the best of my knowledge, TM has never offered a kit to add an extra battery. Several members - perhaps Padgett most recently - have added a second battery. The details of their projects will be found in their posts. I don't recall that any of them have beefed up the floor to handle the extra weight, but I could be wrong. I have visions of the aluminum skin giving way and dumping the batteries on the ground.
The mechanical part of the project is the challenging part. Wiring in the new battery is easy. See, for example,
http://www.trailmanorowners.com/foru...&pictureid=451
for the two common configurations.
Bill