Farmer,
There has been many topics on this subject on this site.
You need to tell us what model toliet you have.
If you have a Thetford that has a china bowl, rotary ball flush valve, for the year model you have you need to replace the seal. Thetford will tell you they don't have an updated seal, but I have found different. The original seal that came with my '05 model had a white coating and the replacement seal had a greenish cast on the seal surface. The first seal lasted 4 months. The replacement has been working flawlessy for over 3 year with FT use.
After you replace the seal use a non caustic cleaner (scubby bubbles)for general cleaning. I will about once a month add Thetford seal conditioner (CW or RV parts stores). We have never done anything else to the seal. HTH
The spell check is IEspell at
IEspell
Once you download then right click and in the box and click on IEspell. Works great and free!