AmEng equivalent for BrEng "decorator"

There is no such word in U.S. English.

If you google "painting and wallpapering", you find a large number of U.S. businesses called "XXX painting and wallpapering". And that doesn't even count those decorators who call themselves "XXX wallpapering and painting", "XXX painters and wallpaper hangers" and other similar variants.

Painter and paper hanger has been a somewhat common phrase in the U.S. since around 1880, and is much less common in the U.K., presumably because they are decorators there.

A person who applies wallpaper to walls for a living is called a painter.

The second definition of painter covers it:

a person whose work is covering surfaces, as walls, with paint

It would be widely assumed that any painter could also hang wallpaper.