Sewer odor, help please

Your grey tank needs flushing. Make sure all your "P" traps are full. Most likely cause is the ASTVD...:B...which is short for Anti-Siphon Trap Vent Device...Its purpose is to allow air into the drain system to help drain the sink while prohibiting gases and odors from coming in from the holding tank and drain piping. They are inexpensive and easy to replace. They are spring loaded with a rubber diaphram. Often this spring breaks or simply wears out. Available at any RV parts store. Also known as a vacuum breaker.....Dennis

Here's a picture of one. There are others, of course.