If it's in warranty, call Thetford and they'll ship you a new valve and seal. Our new Thetford has a very noisy valve. I emailed per another thread and they shipped me a new valve and seal. Had it in a few days. Now I have to install it. Today or tomorrow.
As Dogman said, removing the toilet is simple.
Shut off the water and release the pressure (flush the toilet).
Unscrew the water connection.
Remove the two nuts along the bottom of the toilet.
Lift it off.
You might want to lay cardboard or something over the opening to the tank while you're working on the toilet.