A guy graduates highschool and his parents rent an RV for him to travel the country for the summer and give him a credit card for emergencies. He meets a girl and her younger sister at a camp site and they leave to travel with him. He dates the older sister but has sex with both. He also has sex with other girls and women he picks up and drops off along the way. At one point a guy gets in the RV at a stop light robs them with a knife.
I read this almost 20 years ago and can't remember the title or the author.