Something else you may want to check is your washer drain if you don't have a washer. We kept getting tank smells when we ran our fan and then when we ran the front AC. Here wad of chaulking they had in the pipe to block it cracked. Fixed it and for good measure rubber banded a heavy plastic bag on top. Wa La no more smell. Bout drove me nuts till we found it.
06 Montana 3000RK
07 Dodge 3500 Dually Sport 6.7 Diesel
07 Pearl White Ultra Classic (My new Baby)
10 Cadillac CTS
Texas Boomers---Stop by for a Margie some time!
I used to have the same problem. When I took a shower with the vent fan on in the bathroom the trailer would smell pretty bad. This may be just a coincidence, but I bought a Flush King and started back-washing the grey tanks and haven't had a problem since. Good luck with whatever you try.
I have the same problem in my Jayco any time the fan is on and the door closed and I don't have to be taking a shower. We just have a P trap under the sink and I can feel the air coming into the room under the door when bare foot.