Coach number 777 (a tripple slide H3-45) at Marathon (it's currently on their used page) has a rear bath. It was made for a race car driver who used it for a while before trading up to a newer one.
http://www.marathoncoach.com/preowne...441&coach=0777
It's not very common in XLs because the engine creates a hump in the rear that prevents you from having a flat, non elevated floor. I've only seem them on H3's and I don't really want to know how they plumbed it... I'd like to toilet to be plumbed right to the black tank instead of hanging it over the engine and doing a bunch of complex pipe.