Odor From Sink in Bathroom

Inside sink cabinet on sink drain line after the 'P' trap is an 'air admittance valve'. Hand tightened into a stub pipe...allows air into drain line when dumping waste tank so 'P' traps don;t get sucked dry'

When they don't close.....stink from waste tank gets inside rv

They use cheap rubber flapper held closed by a tang.it stretches and flapper dosen't get pulled closed

Any hardware store for replacement