I'm trying to remember this movie I saw about 20 years ago. It was a science fiction or fantasy horror type film. Like something Steven King would write. The few details I remember are a man in a Midwest type town. In the hotel, an older woman is on the reception desk with a man (presumably her husband) tied up naked behind the desk. The main character visits an abandoned church with something like "abandon hope all who enter" above the door, and an inverted cross inside. At the end the character discovers he is a character in a story being typed on a typewriter by someone.
In the Mouth of Madness by John Carpenter fits the description.
It's about a private investigator John Trent (Sam Neill), who travels to the small town of Hobb's End in New Hampshire to retrieve a manuscript for a book by a horror writer Sutter Cane. Trent experiences bizarre distortions of reality and finds out that Cane is based in a black church in the middle of a town. At the end of the movie, Trent realizes that he was the character in the book.
... movie I saw about 20 years ago
The movie was released in 1994.
... something Steven King would write
While the movie wasn't based on a work by Stephen King, it's indeed very similar to some of his novels, and the plot is centered around a horror novel.
The main character visits an abandoned church with something like "abandon hope all who enter" above the door
Not exactly, but very similar:
Let these doors be sealed by our Lord God and let any who dare enter this unholy site be damned forever.
, and an inverted cross inside