The Fan-Tastic vents do pull a significant amount of air through them and they are probably way to large for just an RV bathroom. We had a similar problem with a motor home, but only when the Fan-Tastic vent was running. Opening the bathroom door resolved the problem. I think it is just back draft from the drains/vents trying to provide make up air. The make-up air has to come from somewhere when the bathroom is depressurized. A vent in the door for make-up air may help. Figure a bathroom with 3' x 4' x 8' dimensions yields about 96 cubic feet (probably much less after removing the volume of the cabinets, toilet, etc) of air in the bathroom. At low speed on a Fan-Tastic vent is rated at 478 standard cubic feet per minute. At low speed, the vent will change the entire volume of air in the bathroom a minimum of 5 times in one minute. Make-up air will be pulled through the opening providing the least resistance.
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