We have recently had the unpleasant experience of a dreadful smell in our bedroom which lasted for several weeks. We have come to the conclusion that it was a dead rat in the roof above our bedroom. The roof above our bedroom is low and flat and we could not get to the rodent to remove it, so we had to evacuate to the kids bedrooms for several nights until the smell became bearable.
I hope you can find some way of removing the possum and blocking it's entry so that it can't get back in.