Good Old Bob always has the answers. LOL.
If the "Bob stink test" dont work mabye someone can be on the roof and listen for air being sucked into the correct air vent as you dump the black tank?
Im sure it would have to have a sucking sound if your close enough to it.
Just an Idea.
2017 Axis / 25.4: Big Foot Levelers, Aims 1200 Watt inverter, Ceramic Window Tint, Full River AGM 6V batteries, Front Hellwig Sway bar / Rear Track Arm, Safe T Plus Steering Stabilizer, PD 4655 converter, Hard Wired Surge Guard, WEboost + WiFi Ranger, LED Headlights/Driving lights, Fresh water surge tank, Black Tank Tornado flush, 4999 zip ties.