I just rebuilt my toilet for the same reason, it is 16 years old. You have to take the toilet out. I replaced the Blade, Blade Seal, and floor seal. To reassemble toilet you need some, I think Poly Brutal sealant. I got it at ACE Hardware. Not to bad a project. Once I got the toilet apart I saw why it was leaking, the minerals in the water put like a crust on the seal over time and the blade will no longer seal.
Here is a link to where I got my parts from.
My toilet is a Aqua Magic, but they carry parts for all types.
http://www.marksrv.com/store52/agora.cgi?&product=toilet%20parts,%20Thetford&user4=Aqua%20Magic%204