Your Fantastic Fan is probably not the problem. Unless you have not followed directions about having a WINDOW open when using the FAN.
Your toilet, after using, opens the slide that drops the goods and the small amount of water down the hatch.
If your sliding seal is not holding water... blame it, not the Fantastic Fan. I doubt if the Fan has Super Powers, mine does not.
Pour some water into the stool... if it drains right away... fix that. If it drains slowly to nothing... same problem. It is leaking water past the sliding gasket that is suppose to hold the water and SEAL the Black Tank gases from entering the interior of your trailer.
The minor weight of the water keeps any vapors from entering the interior of the trailer.
If it holds water and still is noticed. Check the large vent pipe covered by some white thin plywood in an outside corner IN the bathroom. It goes from the floor to the ceiling. Easy to find. Remove the few wood screws and... sniff around.
If this also does not solve the problem, as well. Take a hose, flush the black tank from the inside the trailer, after each trip. Use the Service Station rest rooms as often as possible.
I have other suggestions, but my humor may... stink, as well. Good hunting.