When washing it no longer gets the smell out, then you’ll have to take it a step further with the cleaning solutions. Mildew can be a big issue with something as large as a double sleeping bag and is usually caused by a wet bag not being dried properly. There are other things that can produce a smell that won’t go away like urine and certain bacteria.
When you’ve hit a wall in trying to get this smell out, if instructions permit dryer use, put it in the dryer with a sock full of fragrant herbs. It sounds simple, yet it works much better than dryer sheets. Alternative options other than the sock are a hose or even cheesecloth.
And even if this fails, there is the nuclear option of adding baking soda or a brand named odor remover to the cycle. Be careful with the brand named option, since some of the ingredients can harm the actual fabric.