Had the same issue last season. Fan on or off the blow back during flushing was shmelly!. Using the black tank rinse did nothing. A couple of pails of water down the toilet cleared it up. It moves the solids to the exit. Always try to do the double flush to help the problem.
Married To The Navigator
2005 Red GMC Duramax 4X4 HD Crew Cab
4" Magnaflow, Banks Air Intake
Firestone Ride Rite Air Springs