We bought a 2020 Elkridge 38MB less than 2 weeks ago and have been having trouble getting the sink in the island to drain. It works for a while, then starts to drain slowly, then not at all. We have been doing some troubleshooting, and have replaced the air admittance valve, twice, just in case. The p-trap has been taken off and it is perfectly clean and new. We have snaked the drain under the sink all the way to the tank, as well as snaked the roof vent, with no effect. I used a shop-vac to suck out the gray tank from the bottom to see if there was a clog there, and it worked for about an hour. We have tried to keep the grey tanks closed to see if the sink will continue to drain, no luck. The only solution that keeps the kitchen sink draining is to keep all 3 waste valves open, including the black. This is not an acceptable solution, and I need some ideas about what might be going on.