[Movies] Killer Clown and Ghost Truck

There is a movie I remember watching a long time ago (about 20 years), but am unable to figure out what it is. I was very young at the time, maybe 7 or 8, but I remember quite a bit from that day, everything but the title of the movie. I remember that my step dad was home with us, that he, my sister and I were sitting on the couch watching the movie while my mother spent most of the time in the kitchen making one of her great soups in the large yellow pot that is right now in the kitchen cupboard. More importantly I remember a lot of details about the movie.

I understand that a lot of the details seem to coincide with other movies and I accept that there may be some memory overlap but I am fairly certain that most if not all of these events did in fact occur in this one movie. Here is what I remember of it.

There were a few people, maybe two or three driving down a road in an RV at night. As they were driving, they saw a whole bunch of dead animals laying on the side of the road; mostly sheep. This part happened at night so it is not Close Encounters of the Third Kind. They eventually got to some little town in the middle of nowhere but it seemed to be deserted; there was not a soul in the streets. There was a large, black 18-wheeler tractor trailer (I don't recall if it was just the truck or if the cab was present as well) that kept appearing out of nowhere and chasing them to run them down. It came several times. There was also a sinister looking clown that kept appearing out of nowhere to menace them several times. They found a diner and went in. There were people in there but they acted strange and treated them very oddly—like outsiders—as though the townspeople had a secret. Elsewhere in the town, there was some kind of mystery going on where a detective entered the home of a woman to question her. The room was dark in the middle of the day because the shades were drawn. The woman was lying in her bed and did not get up while she talked with the detective. That scene ended with the detective moving closer and seeing that the woman had a massive lump the size of a soccer ball in the side of her neck (I later learned that that is a goiter). Anyway, the only other thing I can remember clearly is that near the end, the protagonists (the ones who came to town in the RV) managed to trick the truck and the clown and free themselves of both by getting the truck to smash into the clown, crushing him against a wall, while also busting up the truck.

I know that these details are accurate because my sister (who would have been 3 or 4 at the time) has separately corroborated them (yes it was a blind study).

Does anyone have any idea what movie this was?