Do you leave the black water dump valve open and stay hooked up? If so, that is your problem. Or are you hooked up with a leaky dump valve? Anyway you need to keep a lot of fluid in the tank and dump half full or more loads. When we live in our parked trailer we keep the waste valve closed and dump every 5 days or so. Has worked for a long time.
Glad it is not the valve. Maybe fill the tank half full of liquid and take the trailer for a ride on a bumpy road? Maybe use a pressure washer from the outside?