Another vote for the ozone generator but you will probably need to find the cause of the smell, ie; mattress, furniture, carpet, curtains, cooking residue, ceiling cover, walls. After cleaning all those things and provided there's not a leak, vent or dead animal, I think you'd be happy with the results of the ozone generator. They don't cover up smells, they actually eliminate them. Sewer plants are some of the biggest customers of ozone generators.
OH, and don't stay inside until it's aired out for a few hours after using one of them. Ozone is not good for you.