Sewer Odor in Pass-through

[Rick G]

We have a 2016 Montana 3711FL (front living room), immediately behind the living room is the kitchen sink and kitchen, then the bathroom, and then the bedroom. When running a box fan in the kitchen in front of the steps leading up to the living room there is strong sewer odor coming into the camper from the step vents under the steps and there is also strong sewer odor in the pass-through compartment; sometimes the odor is even strong enough that it can be smelled in the basement. Does anyone have an idea of why we are experiencing this odor from the front when the black tank and bathroom are in the back?