[Steamdrivenandy]

You're correct, there aren't too many about. I believe that Khyam make one and a number of Campervan manufacturers offer them eg. Sussex Campervans, Wellhouse, and Lunar initially offered one with their Vacanza (now Camper Car ) model. Failing that, just buying a freestanding 'toilet tent' might suit.
We used a side-mounted Khyam Motorhome with our camper and motorhome, but in the end we got so fed up with attaching/detaching all the time that we ended up just using it as a stand alone tent.