2003 Signature likely came with a Sealand Dometic 3010 Magnum Opus.
Seals are available from https://marinesan.com
. There are two types of seals for different sub models of these toilets. Call them to confirm you order the correct one.
I took ours apart and cleaned everything, applied plumbers grease on the seal and the flapper, and it held water for 2-3 years even with the old seals. I am due to do it again, but will probably install the new seals I have had since then.