It sounds like you have an all plastic Thetford Bravura with a front flush pedal. If you do, You have to pull the toilet to work on the back. There are typically two nuts that hold the toilet to the floor. You will need to pull the plastic cover away from the front of the toilet to access the nuts. You may want to have the water valve and floor seal in hand before you begin the work. The valve is PN 31113. The floor seal is a PN 31115. All this assumes you have a Thetford Bravura. Do an internet search to see if the picture resembles your toilet.