Answer with humor: Park the rig with the bath vent upwind all the time.
Answer with spending your money: If your rig doesn't have a Fantastic-style bath fan, a Heng's, or similar, installing one, and leaving it on would keep the fumes from entering. I can't think of a viable way to alter the tank vent that would be 100% effective all the time. perhaps someone else has had the same experience, and will share a fix.
Every day's a learning experience, though sometimes not what we expected.
2012 Jay Flight 19RD
2016 Ford F150 XLT 2X4 SC 3.5L Eco Max Tow
2010 Tundra TRD DBL Cab (Traded)
1 Spoiled Dog