Wow, what a poor design.
Just so I understand, you have two gray water tanks but only one of them has a roof vent (the bathroom gray water).
The kitchen gray water tank has no roof vent just that sidewall vent? So as the kitchen tank fills there is no place for the air to go but out the side wall or up the trap into the camper.
Yup, that does blow IMHO.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW