The student keeps a journal of his attempts to break the family down and I believe is using it to write a paper. He seduces the wife, takes the young son on an outing at night where he coerces him to vandalize a house with him, etc. When the family starts to realize what he's really like and tries to leave him he loses control and tries to rape the wife in front of her son until they are rescued by the father.
It came out in the late 80's, right around the time of Dead Calm and was of the same genre. It was mainstream but forgettable. The family was driving an RV. The guy they picked up majored in abnormal psych and kept a journal that he narrated from sometimes during the movie on how the "project" was going with entries such as "The wife hasn't had sex in so long I can get her to do just about anything." No physical violence to speak of (except at the end) just psychological, manipulative. More a psychological thriller than a horror movie.