A/C ducting in ceiling is leaking.....either because the duct is just a channel cut in styrofoam insulation or flex duct has hole/tear and black tank vent line is NOT completely sealed at roof line.
Unless you are feeling cold A/C blowing OUT of end of vent pipe vs around vent pipe from 'attic area'
Out the end of vent pipe then pipe is cracked somewhere in attic area
Black tank smells when flushing toilet.........not uncommon
All the poop chemical in the world will not stop that'
Toilet is wide open to black tank when flush ball is open'
Exhaust fan on.....smells pulled up
Exhaust fan off.....smells can still come up
You are NOT leaving black tank drain OPEN are you??
Hope not.....liquids run out leaving pile of poop behind.
Dump when tank id FULL so volume/weight of liquids carries the poop out
Always add couple gallons back into tank so bottom stays wet
If not smelling anything other then when flushing ....no big deal