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I've never seen this, but I browsed listings for horror anthology films I haven't seen and found some possible candidates: From Beyond the Grave (1974), Screams of a Winter Night (1979), Nightmares (1983), Fright Show (1985), Screamtime (1986), From a Whisper to a Scream (1987).
Edited by FloydPinkertonI reread the original post, and I now see you're not sure if this is from an anthology film or not. In that case it could possibly be Big Trouble in Little China, or the 1992 short film Under the Car.
Sounds identical to a segment in the horror anthology film "Campfire Tales" (1997). The segment in question is entitled "the Honeymoon" and involves a couple trapped in an RV with a scary beast outside. http://www.imdb.com/title/tt0115813/

Here's a TURBO-MEGAOBSCURE one...
Long, long ago (back in the 90s) I recall my mother telling me of a movie she saw, which may have been part of an anthology film, in which a couple was trapped in an RV and there was a monster underneath it. If they got out it would kill them, and it was trying to get in or lure them out. I believe, simply based on the idea, the RV was stalled.
Note: because this was in the early-mid nineties, the movie may have come from much earlier, probably the 80s or 70s, though of course it could have been a new movie too or some kind of TV movie or the like. And as I said it may have been a chapter in an anthology film.