Replacing Toilet Floor Flange Seal

This spring I purchased a high rise toilet from Thetford.  It came with a new seal, easy job but yes, once the "hole" was open to the black tank, I about lost it.  Thinking quickly the hole was covered with a piece of plastic and the smell went away in a couple minutes (had the windows and door open).  Did you happen to notice a leak when flushing?  That would indicate a failure of the flange seal.  If not and you are certain the flange has been compromised, cover the hole leading to the black tank and take the flange seal to your local hardware store or something like Home Depot and get a replacement.  I would avoid using caulk.

+1 to Jato re the caulk. The gasket should already be flexible and designed to seal, all the caulk will do is double the number of surfaces that have to stay sealed, plus make it harder to remove in future. If you drain and flush the black tank well first and then plug the hole temporarily while you clean up the mating surfaces the smell shouldn't be too bad.