If you are thinking of installing a household residential style toilet, you'll most likely need to change the toilet flange, that part that holds the toilet in place, where the 2 bolts come thorugh the toilet basin and screw down.
Should be REAL easy in a camper. More than likely, just place on top of your existing hole in the floor and screw it down. Then insert the bolts that comes with the toilet. They have a flat head because they slide in the slot on the toilet flange on the floor. Position them in place, put down the "honey ring", and then attach the toilet on top. When you get ready to install an regular RV toilet again, just unscrew the flange from the floor and put back whatever was originally there and attach an RV toilet.
Simple really. Here's a variety, the flat ones, or the ones with a short neck would probably work quite well. The neck should slip into the hole.
Click here.