do you have (or can you rent) a carpet cleaning machine with an upholstery attachment? if so, try that.
adding 1/3 cup ammonia to the washer (full load's worth of water) after it's filled and started to agitate works to remove odors from fabric. it might work with your mattress. the only question is whether it needs to be rinsed afteward, which would be difficult on a mattress. if you're using a machine, then you'd just need to go over it again with plain water or a regular cleaning solution.
i'd start with a few tablespoons per gallon of water. if it seems to have made a difference, but "not quite", try using a stronger solution.