Sewer smell when using exhaust fans

Thanks for all the suggestions and ideas. We live full time in our RV. Most of the folks here leave their gray line open until it's time to drain the black tank. They don't have the problem we are having. Day before we empty black tank, we close the gray tank, use the deodorizer, then flush line with the gray water. Have tried putting water in sinks when running the exhaust fan that's when the smell comes from under the kitchen sink.