How do I get my parents to stop driving?

My parents both in mid 80s continue to drive a motorhome with car attached. I feel they are no longer able to drive such a large rig. Traveling is their love and I realize that this to some extent keeps them going. Dad has had several moving violations in the past 2 years and one serious accident 12 years ago. His reaction time is very slow and both parents are hard of hearing and have issues with driving at night. It is scary to ride with them. Their past time seems to be driving. I feel they should give up driving the motorhome and limit car driving to local trips only. How do I bring up this subject with them? I fear they might tell me this is what they like to do and will continue. My concern is their safety and the safety of others on the road. Any suggestions on how to handle this would be greatly appreciated.
Thank you,
Dave