As mentioned, a Fantastic Fan running can suck fumes from black tank when flushing. Perhaps something may be causing a positive pressure to build up in the blank tank and being released into the cabin when you flush. A few things to check:
Is there something in the vent stack clogging it (animal nest, etc)?
Has the vent stack dropped, causing the bottom end to be below liquid level in black tank?
Wind is coming from different direction and causing a downdraft in the vent tube (happens with cheap factory vent caps). Shame they just don't put these on at the factory.