we had a stink from no where issue years ago
we found the trap for the washer dryer had lost its water and was letting the odors creep in
now, once a month and sometimes after a days drive
i use a tube and funnel and add water to the drain .
i have also found out, that if the grey is FULL FULL and i open the drain valve it will suck the p trap empty under the shower.
the black and grey share a vent and for some reason if one is really full it will cause a momentary suction on the trap.
plus if its been a bouncy highway drive...almost assuredly the traps are empty enough to allow the smell to say HI