If you're not going to be towing it much, and are going to be hooked up to shore water permanently, I'd put a house toilet in it.
Of these, I like the Toto Drake with the 3" diameter flush valve. Those things could flush a poodle without clogging.
Get the elongated bowl for added comfort and room
If you're gonna tow, I like the Sealand Traveler (by Dometic) with the porcelain bowl. Also in elongated. I have the Sealand in my 34 footer. If you do go with that one, get yourself a backup flush valve to keep on hand. It's easy to damage that valve if you don't winterize properly, and you don't find them in every campground store like you can Thetford parts.
Best of luck,