The drain pipe does indeed just unscrew from the bottom of the black water tank. However, depending on which frame design you have (there are two - old and new, and I don't know when they switched), it may not unscrew because it hits the frame rail as it turns. In that case, you'd have to saw the pipe off and then redo it - it's just PVC, and is easy to do.
The toilet is mounted to the black water tank, and they come out as a unit. Unhook the water line feeding it, remove the screws and clips that hold it to the floor, and lift it out. That's it. Scamp used sealing wax around the drain to hold moisture out, and caulked around the underside of the black water tank. But, in the case of my trailer, these measures didn't work. Be prepared to find a mess under there.
I have replaced my floor, and then sealed the new wood with spray-in bedliner and marine paint
, so that any water that goes under the toilet can't hurt anything. I also plan to use a shower curtain to keep water off the toilet when showering.
Hope this helps!