Googling came up with various pics of the interior of the M113A1 FSV
Best interior shot taken from rear door.
As you can clearly see the middle is naturally enough the gunners position and turret mechanics. The seats are left and right and are taken up by not surprisingly ammo. In some pictures the space closest to the door and to the right actually has a seat belt. I'm assuming it is for the loader. Doesn't look like there is any room in there for anyone extra except maybe a casualty lying on the deck close to the door.