Welcome LP97, I'm glad that your parents can still think for themselves.
Have you read information from cult-exit counselors like Steve Hassan's books (i.e., "Combatting Cult Mind Control", "Releasing the Bonds: Empowering People to Think for Themselves", and (his latest book) "Freedom of Mind: Helping Loved Ones Leave Controlling People, Cults and Beliefs"), visited his website www.freedomofmind.com, and/or watch his FREE videos on his website? Learning about Steve Hassan's BITE model and his methods to help people to critically think for themselves, may help you to communicate better with your parents.
Instead of discussing your doubts with your parents, have you tried to ask them simple questions when they say thought-stopping platitudes and attempt to justify their opinions using something they learned from the WTBTS? It is better to help them to critically think for themselves rather than do the thinking for them. Have you tried getting your parents involved in some time-consuming, non-WTBTS hobby or activity that they love and would help them to meet more non-JWs?
Peace be with you and everyone, who you love,
Robert