Memory alpha mentions a toilet in the Star Trek brig (prison cell):

http://en.memory-alpha.org/wiki/Brig

especially in Star Trek V: The Final Frontier.

![enter image description here][1]

There is a whole separate [toilet][2] entry in Memory Alpha however the toilet in the brig in Star Trek V remains as far as I know, the only visible representation of a potty on the series.


  [1]: https://i.sstatic.net/jIoOG.jpg
  [2]: http://en.memory-alpha.org/wiki/Toilet