I have a 2007 25' cruiser. My toilet sits on top of a plastic black water tank. It measures 16" deep X 18" wide X 7" high which is 2,016 cu in. Where as a gallon is 231 cu in my tank is approximately 8.7 gal. I believe they call it a 9 gal tank for all practical purposes. Hope this helps.
Toilet, Sealand 510, w/o Hand Spray, High Profile - White - Dometic - 302851001 - Parts
www.campingworld.com
We converted it to an Airhead (using the macerator pump 12v tap as the Airhead microfan exhaust) with GREAT RESULTS!! NO Smell ever, and no pump outs ever again!
Consider it, we resisted for over ten years, but now would NEVER GO GACK!
John