I don't have a Casita but in my Sunrader bathroom the top section of the toilet drain flange screwed into the fitting that was underneath the
fiberglass. It was not just stuck together with caulking, it was actually two pieces which threaded into each other. It might be worth a try turning the top section counter clockwise to see if it gives a bit. If it is threaded that would explain why you can't pry it apart from underneath.