Do you leave your gray tank valve open all the time? Have you tried disconnecting from the park's sewer system, shutting your gray tank valve, sanitizing your tank? maybe the smell is coming from their sewer system.
I don't leave it open I watch the levels and empty it as often as it shows full, keeping an eye on the black tank, which takes a while longer to fill up. I make sure the gray is full prior to empting the black, keeping the gray valve closed between emptying it. I wash dishes only and have sink strainers to catch the food debris