"In the room is one window" is technically correct though I think this construction sounds a bit unusual. Except maybe in the context of a list of all the things that are in the room.
I would use this construction before "In the room there is one window." in the context of describing a room. "The room is quite small, no bigger than my bedroom. In the room is one window and two doors. On the walls is very little decoration, save for a single poster." <- example description
What difference does leaving “there“ out make? To my non-native eyes “In the room is one window” looks really strange (tbh, it doesn't seem grammatical, but I'm apparently missing something).
Native speaker here. It's... grammatically correct, but I wouldn't ever say it.
It's grammatically correct, and I talk conversationally like so. I'm American, if that makes a difference here.