If a family have lived in that one room constantly for a whole year it will be full of 'human dander', and I am certainly not being disrespectful of the family that you hosted.
I would take down the curtains and wash those, remove the carpet and dump it, dump the mattress, dispose of all soft furnishings/bed linen/lampshades etc, check the floor boards for signs of new staining, wash the walls and paintwork, check the insides of drawers, cupboards etc., you know your home and how this room looked previously so anything 'untoward' should be obvious to you. It is possible that they had a spillage which they cleaned up themselves and innocently sprayed some perfume around to 'freshen the area' - but stale perfume, regardless of the quality, does smell awful.
Only after doing all the above would I consider redecorating, replacing the carpet and buying a new mattress.