What are you using in the black tank to lubricate your pull handle valves?
(and please... no comments from the "you don't need chemicals in the black tank" group.)
Do you just pour that down the toilet ?? Let it sit in the toilet bowl a few hours and then flush it down into the tank.
Anything that says it's a black tank product that lubricates valves... is gonna work. I use odorlos from Camping World Catalog Item # 58996 . Amazon has it too.
Kathy