what about "इस घर में एक कमरा है"?
That would translate to "This house has one room" or "There is one room in this house" The biggest difference is that इस means "This" so you're now talking about a specific house that you're likely currently inside of or next to.
Is "एक कमरा घर में है" correct? Or is that more like "one room is in the house"?
Right, "One room is in the house" is the translation for that.