What you're looking for is known as TravelLav and was designed by Angelo Colanna back in the early 1950's. If you do a google patent search, you can find the original design documents.
I was very interested in this system and was planning on using it in my conversion until I learned that my girlfriend wasn't into showering where she does other activities.
In my research, I learned that these were possibly used in some rail cars (although I've never actually seen one in that application) and they were used in almost all Flxible bus conversions that were performed by the Custom Coach company (which is still in business in Columbus, OH). From what I can tell, Custom Coach actually bought out the company when they were going out of biz, but didn't do much with the assets of the company. I've talked to Kerwin Elmers, who is one of the founders of Custom Coach and they didn't have any available.
It is available in two versions, one with a drop down sink and one with a fixed sink. Here are shots of the fixed sink version:
I have located one of the fixed sink versions that I could purchase at a reasonable price. I'm not quite willing to let go of it yet since the layout of my coach hasn't been finalized and it's what I'd really like to use. If you're not in a hurry and can wait 6-8 months, then I'll know if I'm willing to give up the one that I know of.
Here's a few shots of another one:
The second shot is looking straight up from below and the next is looking horizontally from below the unit.
In general, look for any Custom Coach Flxible bus conversions and you'll most likely find a TravelLav.
I have a friend who owns one and he said it was prone to clogging... but he has an old bus and hasn't taken apart the whole thing and cleaned it or had anything sand blasted, etc. I'm assuming they'd work great if properly maintained since Custom Coach used them for so many years... I'd think they would have given up on that system if they were not reliable.
Let me know if you're really committed to finding one and then I'll send you what I know and any known sources... although you might have to buy a whole bus just to get the toilet you desire.