Good day. 2015/16 Bullet 269RSL. Living full time, and newbie. We were informed, to leave the gray tank valve open. The kitchen sink is a one basin style large sink. When the sink is half full of water, then drained, I can hear it thump and rattle the pipes, and a few times, stuff came up thru the shower drain. If I took off one of the "vacuum" valves by the sink, it would drain normally. I was then informed to keep the gray tank closed, and drain it when it gets full, so as to prevent stuff from building up, and to allow it to go down. Any thoughts would be appreciated.