We are in the process of bringing an AS that was in a storage facility fire back to life. Our cushions were stained from fire suppression materials as well as from mold and mildew from water that seeped into melted fantastic fans and the melted skylight.
What I did: Took the cushions out and exposed to sunlight and heat. Then vacuumed up the debris and spores. Next, I sprayed them with vinegar, more sun exposure. Then, sprayed them with fabric upholstery cleaner, scrubbed the bad spots, used the vacuum brush the remove dried matter. This removed all the stains and made the fabric look pretty darned good except for the regions exposed to the fire suppression fluid, which unfortunately had a bleach like effect leaving slightly lighter spots with a reddened halo. Since you do not have that to deal with, you should have better luck in improving the look. If the mildew and mold were there for very long, the actual fabric fibers, if natural fiber, may be weakened and eventually wear through quicker than the rest of the upholstery.
I considered purchasing new, which is about $2.5K for a 25 ft dinette lounge. However, that is a chunk of money that I am hesitant to spend, as these now look so good.