toilet problem?

I have a camper that's been sitting unused for a couple of years(i have 2)and I went to get it ready to sell and the toilet won't hold water.I worked the seal with my hand and then scrubed the seal with a toothbrush trying to get it clean and flexible, then coated it with some lube. Still leaks, you can hear it dripping. The disc or whatever you want to call it looks pretty worn, it kinda has a dip in it on the leading edge as it closes. Any suggestions? How do you replace that seal if I can't get it to stop leaking? Or the whole thing.