Subtle difference. "यह कमरा मेरा है" is more "this room is mine". Just to illustrate the difference, when you are asked "Whose is this room/यह कमरा किसका है?", your reply would be "यह कमरा मेरा है". On the other hand, when you are giving a tour of your house and you get to your room, you would say "यह मेरा कमरा है".

Of course in spoken language, you can convey either meaning by placing emphasis on different words (in both Hindi and English).

यह/ये is when the object is on its own (मेरा कमरा). इस/उस is the oblique form for when the object has some postpositions (मेरे कमरा की दरवाज़).

But I'm actually not sure when we use इस and when we use उस. A mother tongue speaker would have to shed light on that.