All I do is open the carage at least once a week and air the place out. I have tried cleaning, chemical etc. The black tank still has some residual stuff left over.
I just finished replacing dump valves becouse Airstream during assemby droped a toilet holding bolt into black tank.
When I had it apart I replaced both valves. It schould be good for the next 20 years.
Regards from Russell in Sunny and cool Tucson Az.