One possibility is that you saw a C-Dory 25 with the tank under the V-berth. I think certain years had it there. I know that Dora~Jean has some photos in a gallery where he cut an access and then marked the tank (in a 25).
I've just been mocking up tanks here and there in my boat (because my original tank sprung a leak, so I decided to get a new tank). For now I think I've decided to go back in the same place. At first I was waffling, because a custom tank was not going to be much more than a new "box" tank, in which case I had a variety of possible locations; but then I found a place that makes the box tanks at a much more economical price (thanks to the archives here), so that made the choice more clear. I still haven't decided between 20 gallons and 27 gallons (I like to swim/rinse).
I thought about putting the tank ahead of the porta-potti area in the V-berth, but decided I wasn't sure about having that much weight that far forward (I can see the plusses to that location though).
Sunbeam