We have a 2021 North Trail 22FBS. Since the very first shakedown trip we have not been able to drain the shower without the help of opening the grey tank. This happens regardless of how empty or full the tank is and we are level. The dealer swears up and down that it did fine for them with a "flood test", but they were able to get a studor vent for the shower covered by warranty installed with some coaxing from me. Now it does this hit and miss again regardless of tank level or even empty but always level. Another possibly related symptom is that when doing a large volume of dishes in the kitchen sink the shower tub will sometimes partially fill up and burp every so often to let the water down, and yes again the grey tank is empty and it still does this. Any thoughts would be greatly appreciated. As of right now the only way I can get the shower to drain is to open the grey tank valve which requires me to always be either at a site with sewer or have my tote tank attached.