An RV dealer near to us sells a "potion" that is supposed to restore the seal. You pour in ¼ of the bottle (a bottle looks like it holds about a pint and a half) and let it sit for 8 hours (or more) without flushing.
The claim is that this will soften and rejuvenate the seal.
However, nothing will replace pieces of the seal if it is damaged, so it might be worth getting one of those fine latex gloves on and see if you can feel if the edge of the seal is smooth or not.
I have not used this product, by the way, so I can't make and assertions of my own as to what it may or may not do...
“Courage is being scared to death, but saddling up anyway.”