Mine was to wide for my 99 dodge diesel and also not high enough so I screwed and glued 4 2x2s to the existing floor and also insulated with Styrofoam in between, helps keep it warm. Mine came with a aftermarket black water tank the was strapped to the bottom, a little micky mouse so when I took a few inches of the back I installed a 25 gallon holding tank an a low profile biffy it works for me.
In Baja last winter I got stuck and my Alaskan is now 24 years old with a little soft wood here and there, so its still in my shop getting a redo on the bottom section. While its there it will also get new o rings and a fluid change. Its not a priority yet and I work slow but will need it in a month or so and will have to get to work soon. I have had my Alaskan since 2004 and in all that time have not found anything that meets my better.