While I haven't had to remove that toilet flange on an RV did have to do it in the house. Using that Dremel or, better yet, a 1/4" chuck capacity die grinder, cut the top off then make one cut down the side of the fitting. By doing that, you may be able to break the rest free and slip the stub off easily with a screw driver as that cement will often break loose. Your replacement will need to be a black ABS or RV type fitting as you already have on your RV. It will need a dedicated ABS cement. A note on that cement - the combo PVC/ABS cement just isn't very good and may break loose due to driving vibrations.
Good luck and let us know how you make out with the project
