Welcome on board!
It looks like your original layout would have been a rear bath, with a gaucho (pull out) bed behind the drivers seat, and then a kitchen area between that and the bathroom. On the passenger side would probably have been a dinette (between the passenger seat and the door, and then another sofa between the door and the bathroom.
That said, all the tanks have probably been removed and you have a blank canvas. Just keep in mind, you will need to position the kitchen where the shorter (24") window is (to give room for the counter/worktop).
Easiest way to work out your best layout is to generate a blank floor plan (using Photoshop or similar) and then make outlines of the beds etc and simply move stuff around.
Fred has a few examples of the 24 Excella in his website, http://www.viewrvs.com/motorhome/air...excella-24.php