i have a similar set up in my CT 13' that you are interested in. its an older dometic (911-M28 - see below), and like Dan, it bolts directly to the black water tank. the PO did the mod so its new territory to me (haven't used it yet since we bought our UHaul
a couple of months ago), however he did a decent job. simply cut a square directly below and epoxy'ed the heck out of it. its a good seal and all only the dump valve is directly underneath (unlike the fancier eggs here with the 90* elbow that brings the dump valve to the edge/side of the trailer for easier access. on the other hand - the straight shot and shorter travel distance will likely ensure a fairly thorough evacuation of the back water tank. i have posted photos here (search Bald Eggle), but if you would like more, pm me and i'm happy to share whatever i can.
having the toilet is such a huge luxury for us ... we cant wait for our trip to Yosemite!