[Stripit]

Since you are stationary have you got up on the roof and run the garden hose dow through the air stsck vent to ensure no bird nests have been built? Or even leaves that might have fallen into that stack and blocked air escaping? You can run water down that pipe as it will go directly into the black tank and clear that line should anything be stuck in there. I used to own the Flush King and back flushed the grey and black tank at least every month or sooner depending on how much time I wanted to expend.