Dying Room Only(1973)

Bob (Dabney Coleman) and Jean (Cloris Leachman) are a couple driving cross country.  They stop to eat at a desolate diner in Arizona.   The owner of the diner, and the one customer, Tom (Ned Beatty) are hostile and rude for no apparent reason.

They go into the rest rooms, but Bob never comes out.  While Jean is calling the sheriff, someone drives off with their car.  Tom suggests that her husband was ditching her, and the sheriff seems to agree.  The sheriff shows little inclination to search for Bob.  There is a rundown motel by the diner, and Jean insists on staying there, although she is clearly not welcome.

During the night, Jean searches and discovers a secret back door to the men’s room with a light to signal the person in there.  The sheriff returns and reveals to Jean that there have been other suspicious disappearances.  The sheriff takes Jean out to the desert where he thinks they are holding Bob.

While they are driving, he calls for backup.  We learn that Lou, an escaped convict is hiding out at the diner.  When Tom or the owner signal to him, he abducts the man in the rest room.  They rob the man, and make his death look like a car accident.  When the sheriff and Jean get there, Bob is still alive.  The sheriff shoots Lou.  Tom shoots the sheriff.  Tom and the diner owner decide to kill both Bob and Jean and make it look like another car accident.  They will plant the bodies to make it look like Lou and the sheriff killed each other.  Tom and the diner guy will act like they were never there.

Jean quietly unties Bob.  Bob lights a flare to burn one of their captors and get the gun.  The backup officers arrive.  The bad guys are arrested and Bob and Jean continue their way home.