Problem solved, but we're not sure how!! Husband checked the roof vent and there was no obstruction there. We unscrewed the air admittance valve under both the kitchen sink and vanity and didn't see any problems there. We made a batch of margaritas and assumed we'd have to go home the next morning and take it in for service. In the morning, my husband pulled the grey line valve and everything flowed out just fine. So .... I'm thinking maybe it was the margaritas??? Thank you to everyone here for their suggestions!
2014 Georgetown 329DS