Are you operating the fans sucking air out of the coach and creating a negative pressure drawing in the odors through the pipes?
Still should not be if there are water traps on the trains to prevent this.
They also sell lemon scented and other additives for the gray tank but they just sorta hide a problem.
We always flush our tanks whenever leaving, or most of the time,a site for next destination or if sitting for a while.
Wife does not allow grease into the gray tank which can cause all kind of wicked smells as it deteroriates and potential tank drain problems.
Even if she did we take two showers every day with liquid shower soaps with nice smells, and wash clothes every three days at least with detergent, that smells good and softeners. Final flush is gray after black tank and the sewer hose smells great as does the tank, not that I sniff them.
If you have loose birds nest etc blocking the vent tube exiting above the coach, which I don't see how could happen, I had someone tell me he reversed the three fans to pull in and closed up all the windows etc and the pressure was enough to blow out the junk. The only exits for the pressure would be though the shower drain and other drains and out the tank vents. You would have to open the valve in the toilet of course.
I'm not vouching for this, or attesting that it will work.Perhaps I shouldn't even mention it. Just another hare brained idea perhaps.