Sewer smell problem what to try next

We have a sewer smell when we leave the gray valve tank open. I have changed the air admittance valve in the bath room and made sure all p traps have water ( sink and shower) no smell in the kitchen. It does not smell when gray tank valve is closed. This just started this year I used to leave the gray valve open all the time last year with no problem . Any suggestions on what to do next other than leave to gray valve closed?
Thanks